No, not lolcats. And I'm not really looking for a specific site, either.
I have often wished that I had some funny picture to illustrate a presentation, a website, a post, an email, or something else. Google image search and stock photo services have hardly ever helped me, although that may be because I'm doing something wrong.
Jeff Atwood seems to have no problem to find funny pictures for his codinghorror and stackoverflow blogs, as well as for the error messages on the trilogy sites. One of my favourites was this elephant. Other bloggers also seem to be quite good at it.
I'm wondering if I simply lack the creativity or if there's sources or methods I don't know about. I could think of the following ways to get pictures, but I'm not sure whether this is really "how they do it".
keep a collection of pictures that you stumbled upon and liked (takes quite some time to build up to a proper library), when you need a picture, there's one in there
maybe have pictures on paper, too, like from magazines or ads.
when you are looking for a picture, search online (Flickr, Google, stock photos). This has never really worked for me, I don't know why.
produce the pictures yourself, i. e. have a good library of source material or find some online and apply some creativity and suitable software. I could imagine that this could work well once you have the practice.