Hi Jarkko,
There is a table called "pa_package_root_folder_map" which links the photo-album package_id to a folder_id, which I imagine you could then link to the photos contained in that folder. (I'm not sure why the package_id column of cr_folders was not used. Perhaps the author was not aware of its existence.)
What happens if you don't include the "url" parameter? Or is it required? If not, maybe it could include a random photo from any installed photo album site-wide or subsite-wide. Just a suggestion.
Thanks for taking on this work...