Add an image style for user pictures

This is a follow-up to #866484: Node user pictures should not be shrunk down via CSS.
Using thumbnail (100px square) for user pictures is not ideal, and if we can all agree on a reasonable size, we might be able to get a fourth default style in.

Keep in mind that this would display next to posts, comments, and on the user page. IIRC they currently display as 100px on comments and user page, and 20px on posts.

Also, this should probably go under image.module, but it directly related to Bartik at the moment, feel free to move it.