Formerly I used SnagIt to add captured images "inline" to my Thunderbird e-mails - i.e. not as a formal attachment.
Using the fantastic Greenshot, I found, that it did not work with the default settings.
However, I want to report by filing here, that a change in default
* Setting -> Expert -> 8-Bit-Bilder erstellen bei weinger als 256 Farben
==> change from enabled to disabled
results in properly formatted and included images in Thunderbird 17.0.
see <img src=http://i.imgur.com/tufTB.png>
The problem is solved for me, but I suggest you change this setting in coming versions to: disabled as default.