Take a look at the existing expansions, it seems pretty straightforward to me.
For a community mod, put all its 1024 x 1024 pixel images in a directory, rename ".png"-s to ".pna"-s, and ".jpeg"-s to ".jpa"-s. Other than this, you'll need an ".ini" file in that directory. This is a plain text file, editable by Notepad, and the files in already existing expansions seems to have lots of comments and explanations. Basically you'll have to list the images and their meta (prices, types, traits etc.) under "[Photo]"; "id" must be the same as the directory's name, "idrange" must be a numerical range, unique compared to other expansion sets, and that's about all to it. I would suggest to copy one of the existing .ini files, and edit that to your needs.