I had the same problem. I fixed it by going into the KDE Control Center. In my version, I select KDE Components, then Component Chooser. Select "Email Client" from the list of components, then select the "Use a different email client" radio button. For the command, you have to add the right options. Mine looks like this (Thunderbird runs from ~/bin in my setup):
/home/steve/bin/thunderbird -compose "attachment=file://%A"
The "%A" is replaced with the file name. I can only get this to work with one image at a time, though.