A comrade has provided a presumably more long-lived link here.
On my Kobo Clara 2E, putting them all in .kobo\screensaver\ causes the reader to load a random image every time I turn it off. Probably works very similar for other Kobo readers.
Non-permanent links: link1 link2 link3. If all the links are dead at some point, I can upload again.

Honestly stolen from Paperback Paradise, @paprbckparadise@twitter, paperback-paradise@tumblr