Unfortunatelly, I still do have some problems with album art under some circumsatances. Imagine I have a folder full of mp3s with a jpg file representing the cover of the album. The id3-tags contain a link to the file. If I delete the link from id3-tag and embed a new image into the tag, MM4 does not show the newly embedded image as album art (cover). The image source folder is somewhere else, but not in the folder containing the issued mp3s. Wondering why the album art seems not to be written to id3-tag, I opened the id3-tag editor of AIMP3. It shows me a very colored but wrong image. It seems MM4 does not embed the image to ID3-tag completly - it seems corrupted. If I use AIMP3 to embed the album art, it is shown in MM correctly, too.
This issue does not always occure. Sometimes it works very fine, but sometimes not.
If I I fully delete the ID3-tag using MM4, close MM4, clean the image cache folders, open MM4 again and embedd the album art again, it seems to work.
Sorry, for only these few information, but I did not find out more, right now.