If it's the entire email not showing in HTML (appeared in 5.0.5) fixed for 5.0.6:
Bug 27033 - HTML mails render as plain text intermittently >
Bug 26432 - When Searching, first result is always displayed as text and not rich text
If you're talking about just embedded images but the rest of the HTML displays properly-
Fixed in 5.0.6:
Bug 27081 - Embedded images are delivered broken >
Bug 27118 - Inline attachments render only 50% of the time >
Bug 23442 - Photos not showing up in body of email
>
Bug 21959 - Inline image attachment not showing correctly
Open about inline embedded images-
Against 5.0.5:
Bug 27904 - inline image problem with Firefox &
Bug 27310 - inline attachments: Can attach only one image as inline at a time
Against 5.0.3:
Bug 26497 - FF2.0: Images in message body not shown
Against 5.0.1:
Bug 24030 - inline image attachments are having 2 src attributes
Against 4.5.5:
Bug 18423 - inline images in mail messages do not display
(Most probably need to be marked dupes/cleaned up)
I would shy towards holding of till 5.0.6 GA on opening another 'inline images won't display' bug unless you grab current source from the FRANKLIN branch and test; else it's kinda too many possibilities to account for right now to find the cause.
Go ahead and fill us in on the bug you're seeing though, and I can check if it appears resolved. Of course, certainly open one right away after 5.0.6 comes out if whatever situation your encountering is reproducible.