This is how you do it on the mobile version of the site.

You scroll down to the bottom of the main menu and press My Profile.

This will lead you to a page displaying the profile information you've entered into the site, followed by a list of options: Edit basic profile info, Edit other profile info, Edit note, and Upload photo.

You want to click on Upload photo.

At the top of the new screen that's loaded, there'll be a button that says "Choose file." You want to click that, and then choose the photo you want to upload to the site. It'll take a moment for the photo to upload to the site.

Underneath the button that says choose photo, there's a list of submission guidelines. You don't need to worry too much about that.

Underneath the submission guidelines, there's another button that says,"Upload your photo." Once you've selected the photo you want to upload to the site, you want to press that.