smugmug is a really cool photo sharing site offering slick web galleries, unlimited storage, the ability to buy prints, mugs, t-shirts and a host of other cool things with your pictures on them. You can even sell your photos through them and use them as a shop to make some money from your photographs and do lots more besides. Enough of the sales pitch (I'm not on commission), but the main thing is if you or a friend / family member has a smugmug site, you can use John's Background Switcher to see those pictures!
You can choose to log in anonymously or with your email address and password (if you have your own account). From there you can either choose a smugmug user's account or use your own respectively and then pick photos at random or from a specific album. Note that if you log in anonymously and pick a specific album and it's password protected, you can enter the password too (likewise the site password if the owner has set one).
Since you can create as many smugmug photo sets as you like, you can select pictures from as many different smugmug accounts as you like!