ff3se 2.12

I can confirm this now as well, today I added image 203 and then saw it messed up the Crusader sprite(s), which besides the imp are the last image. I did some testing by erasing a really large sprite to free up bytes again and sure enough was able to add a bunch more images... until it hit 25,000 free space, then I saw it mess up the 2nd Crusader image. Then I did one more and the 1st Crusader image met a similar fate. Like you said it is something with the total bytes and not the image#. Importing a larger sprite over a smaller one still works fine I'd imagine, but for whatever reason at around 25,000 bytes left it will begin to break the last images upon creating a New Image.

EDIT: Actually importing sprites has the same issue, they break the final images as well upon going under 25,000 bytes. So yeah, 25,000 unusable bytes basically

It's also a bit buggy how the free bytes are reported.  I previously had 29,488 bytes free and could not import anymore.  However, I just now spent the last few hours re-importing all the sprites, including the official sprites.  I also did some re-assigning of the palettes.  Now all of a sudden it says I have 25,392 bytes free with all the same graphics re-imported.  Very strange stuff.