It seems thumbnailing of files with question marks in their names in broken on Commons as of recently. See discussion here:
Some example files:
The error message I get when trying to view one of the broken thumbnails is:
Error creating thumbnail: convert: unable to open image `/mnt/upload6/wikipedia/commons/9/94/D\?K.png': No such file or directory.
convert: unable to open file `/mnt/upload6/wikipedia/commons/9/94/D\?K.png'.
convert: missing an image filename `/mnt/thumbs/wikipedia/commons/thumb/9/94/D?K.png/600px-D?K.png'.
I'd suspect the recent deployment of r65467, but have not been able to reproduce the bug on my local wiki so far.