I dunno where the right place to suggest features is, but have you considered including mood theme hosting as a paid/premium paid feature? LJ (where I have two perm accounts) is actually the only imagehosting site I use -- I don't have whatever one would need for hosting a moodtheme.

I'd adore the ability to just upload a moodtheme all in one place and not have to try to figure out an imagehost solution and have yet another account somewhere.