Since the feature has been introduced into MJ that cover images in the MP3 file's directory are automatically used as cover art MJ ignores cover images in the ID3v2 tag of files.
To reproduce this:
1. Install MP3Ext (
http://www.mutschler.de/mp3ext/MP3ext.html)
2. Put a MP3 file and the cover image called cover.jpg in a directory
3. Invoke the MP3 file's properties, go to the MP3Ext page, and add the cover image to the tag
4. Drag the file into playing now of MJ 8.0-370 and invoke the properties -> MJ will show cover art as "linked to external file cover.jpg" and not as "inside file" as in earlier versions
5. If you remove the image from MJ's properties dialog it will remove the image from the ID3 tag as well! But if you then add the image with MP3Ext again WITHOUT removing the file from MJ and then invoke the MJ properties it will show the image location correctly as "inside file".
The important thing is that MJ has always worked without a problem with files tagged by MP3Ext. The behavior above appeared when the "intelligence" to use cover images from the file's directory had been added to MJ.