Where are you, or what are you doing when this happens? Software provided with equipment like scanners tends to be designed to work with specific email programs (usually Outlook and its relatives) and may not work with Thunderbird. It will ''always ''work if you do it the other way round; start writing your message then add the picture(s) to the email message. Note that this way you can add multiple images to a message; I am not sure that the "send to" option can do this. Thunderbird can set itself as the system default email client. So, in Thunderbird: '''[http://kb.mozillazine.org/Menu_differences_in_Windows,_Linux,_and_Mac/Menu_differences_in_Windows,_Linux,_and_Mac Tools|Options]|General→System Integration'''. This isn't always enough. Your operating system has its own settings too. In Windows 10, '''Start|Settings|Apps|Default apps''' will let you select Thunderbird as the default email program. Occasionally these settings don't seem to "take". It can be useful to change it to something else, close down and restart your computer and then change it back to Thunderbird.