According to the user, Tuxpaint can write files at those paths.
Tuxpaint can read the file currentid.txt which most likely is located at a similar path.
SDL_Image can't open the file located at the path described by currenid.txt
This makes me think that either we have a problem encoding/decoding currentid.txt or SDL_Image has a problem with some paths,
If it is SDL_Image, a workaround I am seeing right now is to to open the file via RWops as we do for kpx images.