Free Hosting Sites
Given this is a sticky I think I should say more than:
That's free... and stuff.
But yeah, Imageshack is a good free hosting site, for pictures. They leave your images up indefinatly (unlike other picture hosting sites such as www.uploadyourimages.com which only leave your images up for 30 days), unless they are in violation of the ToS (which most NWN screenies aren't). The only limits are the image size, since it has to be less than 1024 kilobytes; and the image types which are:
.jpeg, .jpg, .png, .gif, .bmp, .tif, .tiff, and .swf
This isn't really a limitation, since the image types are pretty much standard. Also, if you're not good at keeping track of the URLs it assigns your pictures, it offers a registration service; which is also free, where it will keep track of the images you've uploaded for you.
If that one doesn't suit your fancy, for whatever reason, there's also http://photobucket.com