What's a good iOS app for quickly posting screenshots on the web?
There's also the official Flickr app. It supports uploading photos and videos from the Camera Roll. Flickr specifically will require that you already have an account on the service, most Twitter integrated networks (yFrog, TwitPic) will "create" an account when you use it for the first time.

Flickr the site.
Flickr.app the iOS app.

(Native to iPhone only, obviously works on the iPad using small screen emulation.)