Rate My Kitten is a popular website that has tens of thousands of pictures of cats. They don't have to be kittens, though of course kittens can be the cutest of cats. And you rate them. You give them a score on a scale from 1 to 10, and the next picture loads. Basically, that's it.
If you like looking at pictures of cute kitty-cats, some in funny poses, some tenderly asleep, some prowling and exploring, others draped over each other... this is the site for you. If you prefer dogs, they have a companion site Rate My Puppy. (If you like pictures of naked women, pretty and young, try DOMAI, not affiliated but well worth a plug anyway.)
You can send their pictures as postcards. You can join the group, and once you've got a username you can add comments and submits your own pictures. If you join just to add comments, you must have rather vacant days ahead of you, because they're all like this:
KiddyKatWuvver: oooh those are so cuuute!!!
hugqueen: Aww, beautiful kitty :)
If you do want to vote and add a comment, you have to do one, then hit Back as it's loading the next cat. Or I'm missing something dumb-but-obvious.
There is no advertising or ulterior motive; they just rely on the kindness of strangers to support a heart-warming and harmless website. The So Your Image Was Rejected link is a slightly amusing list of things not to do with pictures of kitty-cats. (Is it a dog? Has it been laid flat on the scanner? etc.)