Same here.
Uploaded a batch of 9 OK. Listed as "processing"; later they disappeared.
Re-uploaded a single one from the batch; made it as far as the edit/ and submit to reviewer (pending).
Decided to try the remaining 8. Listed as "processing"; later they disappeared.
Re-uploaded a single one. Listed as "processing"; but it disappeared too.
No messages or warning, just gone like upload never happened. Hate to keep sending the same ones over and over, but without a rejection message, I have no choice but to keep trying, since one DID go through.
UPDATE: Whoa! I just went back to the same page they disappeared from, and a it seems that all of them have re-appeared, and are ready to edit/submit. Pond5's upload-list page may simply have a bug that fails to display them when they're in some intermediate state (between initial upload-processing and pre-submit editing.)