If a big png image applied as album cover, mp3tag lost ability to read tags from this file anymore (till deleting of cover by another 3rd party program).
This bug applied to, at least, lossless m4a alac files 16-bit/44100 and higher.
Image - png, size 22.1 Mb, size 3000x2989. Sample:
http://rgho.st/86JspYd8x
How big is the M4a before you add the picture and how big is after you add it ?
And just out of curiosity why would you add such a HUGE PNG to a M4a.
Why not convert it to for example JPG ?
HUGE png for HUGE vinyl rip.)
It was 603 Mb and after applying image - 623.
I converted it to jpeg after all, of course. Original image was 105 Mb, i resized it.
This is an undocumented feature 
I've implemented an upper limit for MP4 cover art (which is 16MB at the moment) to prevent memory flood "exploits" and the UI becoming less responsive.
I'm not sure whether I want to change this, but I wanted to give you an update on the situation.
Kind regards
– Florian
That's no problem at all, at least for me - i don't prefer png to jpg images, so i could use JPG image as a cover (and it's only 2,47 Mb).
btw, after applying big PNG cover i even couldn't delete it from track, so... and i couldn't read and edit all other tags, that's it.