I like to use PhotoBucket. Visit Image hosting, free photo sharing & video sharing at Photobucket - create an account and start uploading your pictures there. Once uploaded you can grab the code that says 'IMG code' under your uploaded picture and paste it into one of these posts and you're set.
I like to use Imageshack since they don't require you to sign up. However, Imageshack has starting giving random redirects to other sites when you hit the upload button. I may give in and start using Photobucket.