After upgrading to MT 2.63 or so, I lost the ability to have images posted within comments on my blog. Camille and I discussed it way back when and never got past this stupid little obstacle. Well, I finally bothered to check the settings and I've got a solution.
To allow images in your comments in Movable Type, change the Preferences page within the Weblog Config. Instead of "Use Defaults" in the Sanitze Specs, select "Use my settings" and enter the following:
a href,b,br/,p,strong,em,ul,li,blockquote,img src
Rebuild all pages of your blog. Ta-da!
Note: The Help in MT suggests you not monkey around with the Sanitze Specs too extensively. Using my change we are basically set up to allow links, font formatting (bold, underline, etc), line formatting, and images — nothing more. If you muck things up beyond all recognition, switch back to the "Use Defaults" setting and try it again!