wordpress media upload problems to do with firewall perhaps?

Hi all, I’m working on a site that, for various security reasons, has a firewall and only whitelisted IP adresses can FTP in.

I have been uploading images through the wordpress admin from a whitelisted IP no problem but when I hand over the job of uploading to someone else, someone who isn’t publishing from a whitelisted IP, their uploads fail. The uploaded images show briefly in the “add an image” window but quickly dissapear. There is no error message and the alt text shows up on the site. The images also don’t appear in the uploads folder of the site.

It’s my understanding that the image uploader uses http to upload files so this shouldn’t be happening.

I’ve tested this by uploading an image from my whitelist network and publishing the post. I then disconnect from the whitelist IP and reconnect to another network that isn’t whitelisted and go through the same process, this upload fails.

If anyone could shed any light on this I’d be hugely grateful!