The most common reason a pet's photos can't be added is that the photo file size is too big. (Newer cameras especially take really big photos!) If your image upload "just keeps spinning away and won't load" this is likely why! It can also be related to your internet connection speed.
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 accounts that have used http://www.picresize.com which is a free photo resizing tool that even has a "batch" resize option if you have a bunch to resize too.