Is it possible to change an already committed commit message?

[Cristian Marchi]

I've mixed up a couple of modification that I wanted to commit 
separately and so I've committed them together erroneously. Is there a 
way to change the commit message for rev20415 from "Remove a unused 
screenshot." to "Change ppi to 144 for all figures and remove a unused 
screenshot."?

I've tried with [1] but I have not permission to do it.

I'm sorry for making a little mess.

Thanks
Cristian

[1] http://subversion.apache.org/faq.html#change-log-msg