I like for my songs to have high quality covers but I don't know why it doesn't work with lossless codecs like FLAC.
I have not experienced what you are referring to. I have had no problems adding higher quality art to large file size FLAC files.
Edit:
So it appears that there is a limit to the file size for artwork because of the 24-bit metadata block length that the field uses.
For MP3 files, I found the following: "size limit on MP3s depends on what version of ID3 tag you use. ID3v2.2 also has a limit of around 16 megabytes per picture. ID3v2.3 and ID3v2.4 aren't really limited on a per-picture basis, but the total size of all pictures can't go above a 268 megabyte limit for any tag version."
and since Mp3tag v3.18a there should be an extended error message about this fact:
Putting a 16 MB cover image inside a music file is unreasonable. They get displayed when clicking through the library and would cause a lag, and defeat the purpose of compression. Usually you would put high resolution images in a subdirectory and have a smaller cleaned up cover to display.
Most of my songs are in 24-bit FLAC or WAV and each is over 300MB so I don't really have a reason for songs or covers to be compressed.
Isn't it a little vain to discuss the standards?