Cortez is teasing you. There are two ways you can go with this.
1. Upload picture to a picture hosting site and resize to the proper dimensions there.
2. Use a photo editing software like Gimp or Photoshop ( there are others, those two come to mind) and resize picture there. Save and then upload to your album/avatar.
Also, be aware that there are two places on the site that you can upload a avatar. The main site (pipesmagazine.com) and the forums (pipesmagazine.com/forums). PM one of us on the moderator team (Lawrence, uber4merica (Ben), ssjones (Al) or me) if you continue to have issues.
Good luck!
-Jason