Postimage is easy but I had to sign up (i.e. get a password). The downside to me is that they seem to come down hard if you post copyrighted pics, and how strictly do they construe that? Some copyrighted pics are so common they have become public domain more or less and could fall under "fair use." Also, gray areas like screenshots from bootleg concerts could pose problems.
I do like it though for it's ease of use and you can re-size pics with it.