I have an iPhone image and it has white background, I want the background to be transparent so that I can use it on my site which has a blue background.
Easiest way is with a program like photoshop, or gimp. They are both way over powered for what you need, but the last time I used paint, it didn't support transparency. Basically you'll want to select all connected pixels that are white, and delete them. From there, you can save it as a
.gif file. They both support transparency. With that kind of image, though, I would definitely recommend
You would be better off using one of these: http://www.webanddesigners.com/20-free-phone-and-ipad-gui-psd-and-vector-templates-for-web-designers than an image that may be someone else's copyright.
Just because you can find an image on the Internet, doesn't mean it's free for re-use.