A resume of the further discussion: Indy version shipped with Delphi 2009 has serious problems in HTML email encoding, while Delphi XE2's version is OK.
We did not test Delphi 2010 and XE.
This is a different problem.
We tested in Delphi 2009, and it appears that Indy sends some completely wrong data instead of picture data in emails.
The same code in Delphi XE2 works perfectly.