If the file is too large it will show it uploading but it won't show up in the library, as speculated. You'll need to shrink them down and reupload
them.
also, i can only ever upload two at a time max. Once the first image is complete, it just stops and goes to the library even if the other were in the
process of loading, they just get cut off.
if the op is trying to load multiple images at once, this could be a variation of what he is encountering.