If you are using a Gmail account, you gotta active an option that makes you free to login in any external app from Gmail.
Just go to GMAIL Main page > Settings > Fowarding and POP/IMAP
Once you are here in Fowarding and POP/IMAP
Hope that works your u, it work in my case.
This feature seems to be not supported in current version of Pantheon Mail. I tested with embedded image and I can reproduce your problem.
However the current version of Geary 3.32 supports this (via right-click). Pantheon Mail was forked from Geary, if I'm not misteaken.
I would consider reporting this to elementary OS developers as a bug/feature request. ...