Comment 3 for bug 452874

Finally, I've tracked it down and have a work-around!

The problem comes from the fact that I also have evolution-rss installed. Something in there, possibly due to gtkhtml / webkit rendering, stops Evolution from sending emails with inline images or embedded images. Removing this package and completely restarting evolution (using --force-shutdown) means it now works as expected.

Should be flagged as a bug in evolution-rss package then, even though it affects the functionality of Evolution?