I've got my desktop wallpaper settings set to 'load random images' and I've added some local folders on my computer to pick from, and set it to change every 15 minutes.
There's more than 8000 wallpapers in my gallery, yet I see the same pictures over and over again several times a week. Not just the same one, but lots of the same ones. If it was truly random there should be something like 41 days between repeats of a single picture (8000/2*15/60/24).
My laptop isn't rebooted much, I just leave it in sleep mode, so it doesn't seem to be related to some data that's lost on reboot(?)

The randomness has been an issue for over a year, but the function that was added a while ago which supposedly lists the pictures that have already been used so they're not used again seems to have helped a bit; when I add new stuff to the folders it doesn't take long for it to show up, but it still keeps cycling all the old ones as soon as the new ones have been shown.

I'm guessing either the randomness generator is badly seeded, maybe you're reusing a seed so it keeps repeating the same cycle (like putting in the current time, which repeats every 24 hours), or the output of it is truncated too much to select every possible file when the file list is really long, or you've tried to make your own generator without using an equation that avoids cycling the same number space (you shouldn't make your own generators, it's not an easy thing to do right).

Something is wrong with it, could you have a look at it, please?

Hi Tailen. I have a somewhat similar problem.
You might wish to also follow the discussion at:
http://www.displayfusion.com/Discussions/View/randomness-of-image-selection-for-random-wallpapers/?ID=99fe370e-b4cc-4110-8545-07814a75cc1e

Thanks. I've moved my post over on that thread instead.