This setting is in the header of the download request, you would have to edit the headers coming in. Since it is controlled by the server, I do not think this is an easy solution.

• http://stackoverflow.com/questions/85.../how-to-set-the-default-file-saving-name-when-user-click-download-link

You may be experiencing this bug in case a Content-Disposition header is used.

• bug 1180126 - Content-Disposition: inline; filename= doesn't work with Save As

A possible workaround is to open the image in a tab and use "Save Page As" to save the image.