I agree that its not MP3Tag's task to do album art resizing, its far beyond its scope.
Although it would be nice to be able to use a third party tool to do the bitmap editing from MP3Tag, without having to manually, export, edit, and the import back.
For example, we could just use a freeware command line batch utility like NConvert to do the the resizing or format conversion.
I suppose it can be very difficult to implement this for embedded album art (they would need to be exported and imported from tags on the fly before making them available for the third party tool), but it should be easier for album art kept as separate files, if only we could use title formatting for Tools parameters (something like "nconvert.exe -out jpeg -ratio -resize -ratio 300 0 -o %_folderpath%\%album%.jpg %_folderpath%\images\Cover-Front.tif )