As you probably know, Microsoft has a program they call Xbox Insider, which you can join if you want to try out new features before everyone else and leave feedback. Now, a new feature has been unveiled by Xbox engineering lead Eden Marie that makes it significantly easier to create your own Gamerpics.
Thanks to this, you can use both screenshots and Achievement Art and cut out the part you want as a Gamerpic. To make things even better, the resolution requirement has been lowered if you want to upload your own images to use as a Gamerpic.
It is also possible to use the same crop tool to trim the screenshots you want as a wallpaper in the Xbox dashboard to get it as nice (and perhaps free of health bars and other HUD?) As you want it.
We don't know when the feature will be released for everyone, but normally it takes one or two months before news like this is made public.