Having image/html troubles with Thunderbird? RSS account showing brief description instead of the full html web page? This may not be the only solution but it was the one I discovered worked for me and was from my own actions that caused it.
Did you happen to change your email settings to display messages as simple HTML at some stage? If so, turn it back to Original HTML. Hey presto! Thunderbird’s RSS reader displays the full web page again and also, displays images in your email again.
For more information, see http://kb.mozillazine.org/Images_in_messages_do_not_appear