Best thing todo is to create a second table (or add a column to the current table), add a file upload feature to the registration form. When registration is successful add the picture to the db with EntityFramework. Create a page to return the picture by the userid so you can include it in a page somewhere.
Something like this http://blogs.msdn.com/b/webdev/archive/2013/10/16/customizing-profile-information-in-asp-net-identity-in-vs-2013-templates.aspx but with a profile picture