First you have to make sure your image is a web-friendly format - usually .jpg or .jpeg, .gif, and .png are accepted.
Then, upload it to an image-hosting site (I use imgur.com because you don't need an account in order to use it, but there are tons of different ones that would work).
Then, take the image's web address (which should look something like "http://i.imgur.com/blahblah.png") and paste it into the HTML code... I think that's <body background="IMAGE"> but I mostly use CSS these days so I don't know for sure.