Thanks for the reply!
When you hsve the "Also store image in file's tag" setting enabled MC is supposed the replace the existing embedded image. Do you mean that you can see both images in some external application? If yes, perhaps the old image is not stored in a supported way or format. What is the audio file format? What is the image file format? How did you embed the old image?
The music files I tried with are mp3 with ID3v2.3 tags. They had each one jpg file embedded with MP3Tag and I used MP3Tag to see that each file had two images (the original and the one I dropped in MC) embedded after I had dropped a jpg file on them in MC. After the drop the image in the window you directed me to changed to the dropped image. But if I switched to a file with another image and then switched back, the original image was shown in that window and as the small thumbnail in the file list. Below are the tags from one file after the drop:
MPEG-1 Layer 3
190 Kbit VBR
44.1 Khz Joint stereo
Copyrighted: No
Original: No
Protected by CRC: No
Encoder: LAME
Gapless: Yes (576 start, 2016 end)
ID3v1 Tag: none
ID3v2.3 Tag: (69632 bytes)
TALB (Album): Straight Ahead
TPE1 (Artist): Abbey Lincoln
TPE2 (Album Artist): Abbey Lincoln
COMM (Comment): Sörens eMusic
TCOM (Composer): E. Baker; Abbey Lincoln; Mal Waldron
TCON (Genre): Vocal
TLEN: 323336
TPUB (Publisher): Candid
TIT2 (Name): Straight Ahead
TRCK (Track #): 1
TYER (Year): 1961
TXXX (Album Artist): Abbey Lincoln
TXXX (Album rating): 5
TXXX (Mood): Intense; Fiery; Complex; Freewheeling; Passionate; Bittersweet; Autumnal; Elegant; Soothing
TXXX (MusicMagic Data): <too large to display>
TXXX (MusicMagic Fingerp..): <too large to display>
TXXX (replaygain_album_g..): -0.73 dB
TXXX (replaygain_album_p..): 0.951542
TXXX (replaygain_track_g..): +2.01 dB
TXXX (replaygain_track_p..): 0.460050
TXXX (Review): <too large to display>
TXXX (Style): Vocal Jazz; Post-Bop
TXXX (Theme): Sunday Afternoon; Feeling Blue; Late Night; Relaxation
TXXX (Tool Name): Media Center
TXXX (Tool Version): 15.0.73
APIC (MP3T1732716757) (Cover): <too large to display>
APIC (Picture) (Cover): <too large to display>
The APIC (MP3T1732716757) (Cover): tag was there before and the other APIC tag is new. The original image for that particular file was probably embedded with MP3Tag some time ago (or the embedding might have been done by eMusic). When MP3Tag nowadays embeds cover art the file gets a APIC (Image File) (Cover): tag. I also tried with files that had that tag (APIC (Image File) (Cover): ) originally and the result was the same, another APIC (Picture) (Cover): added and two embedded images shown in MP3Tag.
If you have grouping enabled the files are sorted only inside each group. You can disable grouping or create a separate ungrouped view if you want to sort all included files. Regarding sorting by the image you cannot sort the thumbnail images, but you can add the "Image File" column to a list view and sort by it. However, when the images are embedded they all will have the same value (Inside File) and sorting does not do anything. It works as expected when the Image File column shows the filenames of the linked images.
OK I guess I will have to sort the generic images out manually then.