To choose a photo that appears with your username, click on 'user menu', then on 'edit profile', then on 'choose avatar' link, which will open a box full of images to choose from. (IMO, but not whomever is in charge of these things, an avatar should be removed from the list, once it's chosen - if you don't want to share, check to see who is using what, before choosing one. As much as I like my Roadrunner avatar, if someone else chooses it, I will have to find another, and learn how to import it - something I haven't figured out how to do yet...) Choose one, click to upload it, and then click on 'update profile', and your avatar should appear when you post. (You'll probably want to post something, to make sure that it does)
If you want to import a photo as your avatar, someone else will have to do the how-to, because I can't. Good luck!