When you're level 10, go to edit profile and pick a showcase at bottom. You can also do animated gifs if you pick artwork showcase instead. Plus it's easier to get the image file onto Steam with that showcase - with screenshots you have to take a screenshot and replace the file in your Steam folder before uploading.
Will you be able to showcase art
not game related? Like, something from your computer?
When I get to level 10, I'll probably feel the need to showcase anime stuff...