Make sure the filetype is correct. Try opening the file, selecting and copying it (not copying the file itself; just the comprising pixels), creating a new file, pasting, then saving with a different name and as an uncompressed .TGA.
"Call yourself alive? I promise you you'll be deafened by dust falling on the furniture,
you'll feel your eyebrows turning to two gashes, and your shoulder blades will ache for want of wings."
Change both the height and width sizes to 128 pixels
Click on the new window
Ctrl-V
File -> Save as...
Click on "File type (by extension)..." (near the bottom of the window)
TarGA image
Save with a different file name to your previous one
GIMP should open a window containing two tickboxes. Untick the one that says "RLE compression" next to it
Done. This might help, it's worked for me in the past. Also remember to leave the textures up to a couple of hours to upload, as the server may be busy.
"Call yourself alive? I promise you you'll be deafened by dust falling on the furniture,
you'll feel your eyebrows turning to two gashes, and your shoulder blades will ache for want of wings."
Evidently, you haven't checked or heard that the site is bugging and doesn't work at the moment.
"Call yourself alive? I promise you you'll be deafened by dust falling on the furniture,
you'll feel your eyebrows turning to two gashes, and your shoulder blades will ache for want of wings."