So I've had to explain this to a lot of my friends online, since my current gamerpic is the Hydra logo, so I thought I'd make a journal out of it. There's no hacking or photoshop involved, and it's incredibly easy once you know how!
What you'll need:
-A computer with windows 10-
-The Xbox Beta app-
Totally free, and can be found in the windows/Microsoft app store on the computer. The app is only be available to Windows 10 users as of now.
-An image that's at least 1080 x 1080-
A good way to find these is doing a google search for
"(whatever you want) hd" or "(whatever you want) wallpaper" The picture can be about 1080 x 1080, but you wont be able to use it if it's below that resolution.
Sign in to the app like you'd sign in to the normal Xbox app. Go to edit your profile, and then to change your gamerpic. Above the standard list of options, you'll find an option to set your own custom image. There's no limit to how many times you can change your image, but you can still be reported for an obscene image. So please be smart!