A profile pic only shows on your profile.
An avatar is a pic that shows up in the forums as well, such as my eyeball pic.
Using phones seems to be tricky for uploading pics, so i'm not sure if it will work or not. But if you scroll to the bottom you will see a link that says 'Use full site' or something along those lines. This will take you to the non-mobile version of the site.
You will see a line of blue words near the top. One of those words is 'Photo'. Click that and it will take you to the page to add a profile picture. If you'd rather, or also, want to add an avatar, follow the same steps but instead of clicking 'Photo' you'll want to click 'Avatar' on the same line.
Again, not sure if it will allow you to on a phone or not.