I ran pngcheck on all of the Thunderstorm images, they all pass:
OK: ActionWindow_Back.png (192x64, 32-bit RGB+alpha, non-interlaced, 66.1%).
OK: ActionWindow_Close.png (192x64, 32-bit RGB+alpha, non-interlaced, 64.9%).
OK: ActionWindow_Minimize.png (192x64, 32-bit RGB+alpha, non-interlaced, 67.3%).
OK: ActionWindow_Restore.png (192x64, 32-bit RGB+alpha, non-interlaced, 66.9%).
OK: ActionWindow_Template.png (345x388, 32-bit RGB+alpha, non-interlaced, 96.5%).
OK: ActionWindow_Tools.png (192x64, 32-bit RGB+alpha, non-interlaced, 67.1%).
OK: Frame_BottomBorder.png (1662x32, 32-bit RGB+alpha, non-interlaced, 92.3%).
OK: Frame_Button.png (675x80, 32-bit RGB+alpha, non-interlaced, 85.6%).
OK: Frame_CheckBox.png (416x52, 32-bit RGB+alpha, non-interlaced, 74.2%).
OK: Frame_ComboBox.png (256x68, 32-bit RGB+alpha, non-interlaced, 76.7%).
OK: Frame_DefaultButton.png (164x23, 32-bit RGB+alpha, non-interlaced, 76.0%).
OK: Frame_FullScreenArrowsCollapse.png (176x46, 32-bit RGB+alpha, interlaced, 28.6%).
OK: Frame_FullScreenArrowsExpand.png (176x46, 32-bit RGB+alpha, interlaced, 26.7%).
OK: Frame_LeftBorder.png (32x1166, 32-bit RGB+alpha, non-interlaced, 87.3%).
OK: Frame_RadioButton.png (416x52, 32-bit RGB+alpha, non-interlaced, 67.2%).
OK: Frame_RightBorder.png (32x1166, 32-bit RGB+alpha, non-interlaced, 87.0%).
OK: Frame_TopBorder_Mac.png (1662x124, 32-bit RGB+alpha, non-interlaced, 94.0%).
OK: Frame_TopBorder.png (1662x124, 32-bit RGB+alpha, non-interlaced, 94.1%).
OK: FullscreenPlayer_Mac.png (1726x376, 32-bit RGB+alpha, non-interlaced, 98.0%).
OK: FullscreenPlayer.png (1726x376, 32-bit RGB+alpha, non-interlaced, 98.3%).
OK: Groupbox_BottomBorder.png (80x16, 8-bit palette, non-interlaced, -1097.1%).
OK: Groupbox_CaptionArea.png (128x60, 8-bit palette, non-interlaced, -101.5%).
OK: Groupbox_LeftBorder.png (8x100, 8-bit palette, non-interlaced, -1818.5%).
OK: Groupbox_RightBorder.png (8x100, 8-bit palette, non-interlaced, -1818.4%).
OK: Groupbox_TopBorder.png (80x76, 8-bit palette, non-interlaced, -152.6%).
OK: List_BrowseHeaderBar.png (84x96, 8-bit palette, non-interlaced, -91.7%).
OK: List_Button.png (128x32, 32-bit RGB+alpha, interlaced, -22.6%).
OK: List_GroupHeader.png (86x48, 32-bit RGB+alpha, non-interlaced, 5.0%).
OK: List_HeaderBar.png (150x20, 24-bit RGB, non-interlaced, 87.1%).
OK: List_Rating.png (90x32, 32-bit RGB+alpha, non-interlaced, -52.1%).
OK: List_Selection.png (1179x92, 32-bit RGB+alpha, non-interlaced, 94.0%).
OK: MainFrame_BottomBorder.png (1726x120, 32-bit RGB+alpha, non-interlaced, 97.4%).
OK: MainFrame_LeftBorder.png (24x260, 32-bit RGB+alpha, non-interlaced, 35.3%).
OK: MainFrame_RightBorder.png (24x260, 32-bit RGB+alpha, non-interlaced, 35.3%).
OK: MainFrame_TopBorder_Mac.png (1726x400, 32-bit RGB+alpha, non-interlaced, 98.1%).
OK: MainFrame_TopBorder.png (1726x400, 32-bit RGB+alpha, non-interlaced, 98.0%).
OK: Menu_Back.png (38x33, 24-bit RGB, non-interlaced, 86.8%).
OK: Menu_Item.png (786x92, 32-bit RGB+alpha, non-interlaced, 92.9%).
OK: PlayerBar_ContButton.png (333x126, 32-bit RGB+alpha, non-interlaced, 72.1%).
OK: PlayerBar_ContOffButton.png (333x126, 32-bit RGB+alpha, non-interlaced, 71.9%).
OK: PlayerBar_ContSongButton.png (333x126, 32-bit RGB+alpha, non-interlaced, 71.1%).
OK: Playerbar_Display.png (258x75, 32-bit RGB+alpha, non-interlaced, 73.8%).
OK: PlayerBar_DSPButton.png (333x126, 32-bit RGB+alpha, non-interlaced, 70.7%).
OK: PlayerBar_MuteButton.png (471x86, 32-bit RGB+alpha, non-interlaced, 72.0%).
OK: PlayerBar_MuteOffButton.png (471x86, 32-bit RGB+alpha, non-interlaced, 64.8%).
OK: Playerbar_NextButton.png (549x213, 32-bit RGB+alpha, non-interlaced, 83.4%).
OK: Playerbar_PauseButton.png (633x213, 32-bit RGB+alpha, non-interlaced, 82.2%).
OK: Playerbar_PlayButton.png (633x213, 32-bit RGB+alpha, non-interlaced, 81.5%).
OK: Playerbar_PrevButton.png (546x213, 32-bit RGB+alpha, non-interlaced, 83.3%).
OK: PlayerBar_SearchButton.png (112x28, 32-bit RGB+alpha, interlaced, -34.9%).
OK: Playerbar_Search.png (183x44, 32-bit RGB+alpha, non-interlaced, 35.7%).
OK: PlayerBar_ShButton.png (261x126, 32-bit RGB+alpha, non-interlaced, 69.4%).
OK: PlayerBar_ShOffButton.png (261x126, 32-bit RGB+alpha, non-interlaced, 65.1%).
OK: Playerbar_SliderPosFront.png (638x116, 32-bit RGB+alpha, non-interlaced, 94.5%).
OK: Playerbar_SliderPos.png (638x116, 32-bit RGB+alpha, non-interlaced, 90.1%).
OK: Playerbar_SliderPosThumb.png (508x116, 32-bit RGB+alpha, non-interlaced, 90.8%).
OK: Playerbar_SliderVolFront.png (638x80, 32-bit RGB+alpha, non-interlaced, 92.1%).
OK: Playerbar_SliderVol.png (638x80, 32-bit RGB+alpha, non-interlaced, 85.8%).
OK: Playerbar_SliderVolThumb.png (508x80, 32-bit RGB+alpha, non-interlaced, 86.8%).
OK: Playerbar_StopButton.png (423x213, 32-bit RGB+alpha, non-interlaced, 82.8%).
OK: Progress_Back.png (72x124, 8-bit palette, non-interlaced, -72.4%).
OK: Progress_Front.png (72x124, 24-bit RGB, non-interlaced, 42.9%).
OK: Scrollbar_HorizontalBackground.png (192x64, 32-bit RGB+alpha, non-interlaced, 63.6%).
OK: Scrollbar_HorizontalHandle.png (256x64, 32-bit RGB+alpha, non-interlaced, 71.6%).
OK: Scrollbar_HorizontalLeftArrow.png (256x64, 24-bit RGB, non-interlaced, 50.9%).
OK: Scrollbar_HorizontalRightArrow.png (256x64, 24-bit RGB, non-interlaced, 50.9%).
OK: Scrollbar_Stump.png (128x128, 24-bit RGB, non-interlaced, 67.0%).
OK: Scrollbar_VerticalBackground.png (64x193, 24-bit RGB, non-interlaced, 53.2%).
OK: Scrollbar_VerticalDownArrow.png (256x64, 24-bit RGB, non-interlaced, 50.1%).
OK: Scrollbar_VerticalHandle.png (256x64, 32-bit RGB+alpha, non-interlaced, 70.6%).
OK: Scrollbar_VerticalUpArrow.png (256x64, 24-bit RGB, non-interlaced, 50.1%).
OK: Search_Clear.png (128x32, 32-bit RGB+alpha, interlaced, -31.2%).
OK: Slider_HorizontalBackground.png (238x64, 32-bit RGB+alpha, non-interlaced, 71.6%).
OK: Slider_HorizontalHandle.png (176x92, 32-bit RGB+alpha, non-interlaced, 75.7%).
OK: Slider_VerticalBackground.png (128x119, 32-bit RGB+alpha, non-interlaced, 71.4%).
OK: Slider_VerticalHandle.png (336x44, 32-bit RGB+alpha, non-interlaced, 73.7%).
OK: SlimPlayer.png (1726x376, 32-bit RGB+alpha, non-interlaced, 98.3%).
OK: SplitViewTab_BorderRow.png (208x32, 24-bit RGB, non-interlaced, 24.5%).
OK: SplitViewTab_Button.png (620x96, 32-bit RGB+alpha, non-interlaced, 91.1%).
OK: SplitViewTab_Close.png (150x30, 32-bit RGB+alpha, interlaced, -16.3%).
OK: StatusBar_Display.png (200x38, 8-bit palette, non-interlaced, -101.9%).
OK: Tab_BottomBorder.png (40x12, 8-bit palette, non-interlaced, -3090.6%).
OK: Tab_Button.png (216x72, 32-bit RGB+alpha, non-interlaced, 75.8%).
OK: Tab_LeftBorder.png (24x92, 8-bit palette, non-interlaced, -596.2%).
OK: Tab_RightBorder.png (24x92, 8-bit palette, non-interlaced, -596.1%).
OK: Tab_TopBorder.png (80x16, 8-bit palette, non-interlaced, -1096.8%).
OK: TagWindow_FieldLabel.png (190x46, 24-bit RGB, non-interlaced, 98.9%).
OK: TagWindow_Field.png (380x46, 24-bit RGB, non-interlaced, 99.4%).
OK: TagWindow_Group.png (100x22, 24-bit RGB, non-interlaced, 92.2%).
OK: TargetBar.png (800x96, 8-bit palette, non-interlaced, 79.7%).
OK: TitleBar_CloseButton_Mac.png (340x68, 32-bit RGB+alpha, non-interlaced, 47.7%).
OK: TitleBar_CloseButton.png (340x68, 32-bit RGB+alpha, non-interlaced, 47.7%).
OK: TitleBar_MaximizeButton_Mac.png (340x68, 32-bit RGB+alpha, non-interlaced, 62.7%).
OK: TitleBar_MaximizeButton.png (340x68, 32-bit RGB+alpha, non-interlaced, 63.1%).
OK: TitleBar_MinimizeButton_Mac.png (340x68, 32-bit RGB+alpha, non-interlaced, 63.0%).
OK: TitleBar_MinimizeButton.png (340x68, 32-bit RGB+alpha, non-interlaced, 63.2%).
OK: TitleBar_RestoreButton.png (340x68, 32-bit RGB+alpha, non-interlaced, 62.6%).
OK: Tree_Selection.png (786x92, 32-bit RGB+alpha, non-interlaced, 91.5%).
OK: UpDown_Down.png (256x44, 32-bit RGB+alpha, non-interlaced, 65.3%).
OK: UpDown_Left.png (256x80, 32-bit RGB+alpha, non-interlaced, 80.1%).
OK: UpDown_Right.png (256x80, 32-bit RGB+alpha, non-interlaced, 80.1%).
OK: UpDown_Up.png (256x40, 32-bit RGB+alpha, non-interlaced, 61.8%).