Just a reminder about the naming of the file images. Please do not use automated/randomized namings. You can do this even when uploading a batch of pics at the same time. Just look for "Destination Filename" box(es) and replace the name(s) inside with a more appropriate name(s).
Also, Simant is pushing for putting fair use license on the images, please try to put them next time too.
The validation in place is:
for filenames they cannot be all numbers or length less than 8 (including file extension).
the source cannot be blank
and a license should be selected.
If you believe you met those and it still won't let you, provide me with the browser you are using, the filename you are trying to upload, the source entered and the license you selected and I can try to reproduce and fix the issue.