Nothing as far as I can tell.

Both are RGB one version as a JPG and the other version saved as PNG.

Saved from photoshop. Res is 400x300 46Kb and 135Kb. I have also tried other files and they all fail.

Using Firefox 1.5.0.6

Tried it with internet explorer - it also fails.

Not sure what else to try now. I have been able to upload before so I am not sure what has been changed. Why not allow attachments in the normal way. Much easier. No need to be dependent on some third party host computer being available.