How to create a white background to a jpg/png?
I don't know how to do that with Preview.app or iPhoto, but for this purpose you could use an online photo editor. First hit on google here is http://pixlr.com/editor/ - and it does the trick.
(upload the image and you can make it white in several ways, e.g. by pressing B or G after you have set the main color to white, though there wil be more intelligent ways)