The wiki is full of many mysterious things, much of it kind of out of date at this point XD Some quick poking at the source code suggests that what would be easiest is a zip file of all the images, with the file names being the mood they go to. If you have a github account (or are willing to sign up for a github account), you should be able to create a new issue here and attach the files: https://github.com/dreamwidth/dreamwidth/issues - but if not, my gmail account is the same as my DW username!

(also per your other comment, I was poking at the existing mood themes and at least one of them is png so you're good there)