You can't change the afvatar in the iPF app (well, you can but it's a pain). Go to the site and log in with a browser, click on the User CP tab at the top, and choose Edit Avatar from the column at the right.
If you are on the iPad you can choose one of the supplied avatars. If you are on a computer you can choose to upload your own avatar. Pay attention to the size limitations.
It is possible to upload a personal avatar using the iPad, but it's a lot of trouble. Use the computer if you have a choice. If not I'll list the hoops you'll have to jump though to so it using the iPad. Uploading files to web sites is one of the things it's not so good at, unless the site has an app for it, of course.