The most common reason a pet's photos can't be added is that the photo file size is too big. Newer cameras and phones can take really big photos!
Try adding the pet with 0 photos.
If that works, check the size of each photo.
Each photo file size can be a maximum of 8MB
and the formats allowed are JPG, JPEG, GIF, BMP and PNG.
Photos only display at about 100k (that's .01MB). Smaller photo files will upload faster. Try adding big photos one at a time.
If your photos are too big, some shelters use http://www.picresize.com which is a free online photo resizing tool that even has a "batch" resize option if you have a bunch to resize too.