[Neil]

When you add photos Mick, even your own, do you do it through the gallery admin pages?
I think thats why it works for you and no one else because I cant upload either and I have never uploaded a picture to my gallery so the new user bit is confusing.

Its more than likely a php error that probably started after a php upgrade on the server.

If you are uploading pictures directly in the admin pages then I think you wont be using the picture processing that is causing the error when users upload normally.