v2.38 I selected all songs in my list (mostly MP3's), edited Comment (added the text ++my ), pressed "Save" to save the comment changes.
Upon checking my music files, I was horrified to find that Select All Files, edit Comments erases ALL ALbum Cover Artwork !
PLEASE improve on this bug; it's a few hours of hard work down the drain with the lost album artwork... at least I still have the artwork files. Some users won't....
Are you talking about MP3 files? Any chance you are writing APEv2 tags too (you can check this in the options)?
Having used your program before, I literally downloaded your v2.38, installed, and processed my files as described above (there was no previous versions of MP3Tag, as I did a reinstall of Windows-XP-Pro SP2 with IE7.) I stress all MP3Tag settings were at "default" upon install--I definitely did not change any settings.
And Yes, by default your program did write APEv2 Tag...
Tools>>Options>>Tag
and there was a checkmark enabling writing of APEv2 tag...
OK, in case Mp3tag wrote APEv2 tags inside MP3 files, your album art is still there, but in the ID3v2 tag only since AFAIK, APEv2 does not support binary data. Depending on the writing settings, Mp3tag also updated the ID3v2 tag to reflect the changes you made. If that is the case, you can simply remove the APEv2 tags from the MP3 files by only selecting APEv2 in the options under Remove or Delete (don't know the exact name by heart) and then klicking on 
.
To check if the ID3v2 tags contain the correct information, you can also disable reading of APEv2 tags. In that case, Mp3tag will use the tag with the next lower priority: ID3v2.