Get Different Images from one URL

For example,

http://ift.tt/29W8XpQ

is one url. If you reload the page, it comes with a new image. My website has a background image, and I want it to be a random image of two images:

http://bit.do/bgimage1

http://bit.do/bgimage2

How can I achieve this? And the calling of the image in the css.