So far, we've been unable to reproduce the problem you described, even using a Mac with both Firefox and Safari.
At what point does that error message appear? When you click the Upload Images button (which should open a popup window where you can select files to upload), or after you select files when you then try to upload them? Which browser application are you using (Safari, Firefox, etc.)?
Also, please send the Item Name of the product you're trying to edit along with your E-junkie password to us by email: