Every guide I've read recommends using imapsync and msmtp for sending.
http://stevelosh.com/blog/2012/10/the-homely-mutt/#getting-e...
Anyhow, Gnus can render "html" articles; `mm-text-heml-renderer' defaults to the built-in `shr' in Emacs 24.
IME, it works out rather well for most HTML emails I receive.
Are the problems you're describing due to content within the images themselves, or due to placement of the text relative to the image?
K9mail doesn't nest reply threads[1] which is must have feature for me. Otherwise yes k9mail is good.
How is it the best on any platform? I thought it only existed on Android?