Playback of images needs some work, IMO ... before some of the power-user features are implemented.
1. When music and images are both playing at the same time, it gets confusing as to how to control each media. Maybe you could make the top control bar exclusively for music playback... even if there are only images being played. Then, you could make the bottom bar into the exclusive image control bar. It seems logical as it already controls, to a certain extent, the images and visualizations, and there looks to be enough room available on it to add the controls for PLAY, PAUSE, FIRST, LAST, etc. (Maybe you could also add the slideshow START/STOP buttom there, too?)
2. As far as thumbnails go... I'll keep my comments, again, to the primary issue of PLAYBACK. Here, my suggestion would be to make another template for playback... like the Track Info idea. On this template, you could have the customary thumbnail pane on the left (or bottom) with the full image view on the adjacent pane. This view scheme might just be added to the current playback options of IMAGE, COVER ART, TRACK INFO, VISUALIZATIONS, etc. Maybe call it THUMBNAILS or something.
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That said, for a power-user request, I'd like to see a way to send cover art to an external editor without the need to find the image in the images library. It would be nice to be able to send it directly from either the playback view.... or from the right-click menu for the Action Window / File Properties / Image window.
Also, a minor fix might be to make it easier to specify the length that slideshow images display. My slider jumps from 0:01 to 0:08 to 0:15 ... with nothing available in between.
Thank you.