It's possible that auto correct is doing some weird things for you. Try
going through the option I listed in my previous post and disabling
everything.
Also, in the new message window, under the Options tab, at first I'd say
make sure that HTML is selected, but see what happens if you go to plain text.
Lastly, when you paste, click the little clicpboard icon that pops up and
try doing either match source formatting, or keep text only, though the
latter would get rid of embeded pictures, you could try going to insert
picture later, and putting it in the appropriate spot.
Hope something here helps! If not, you may want to try another HTML editor