how do I stop Mozilla from scanning for viruses after jpg download ?

The only setting that controls this is for all downloads.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. Locate the preference browser.download.manager.scanWhenDone and double-click on it to change its value to false

This will stop Firefox requesting a scan, but that may not stop the anti-virus program from scanning downloads of its own accord.

See also "Disable virus scanning in Firefox preferences - Windows"

• http://kb.mozillazine.org/Unable_to_save_or_download_files