So if the image is already on the net, then that's easy, simply copy the url link and paste it here, putting [ img= infront ot it, and ] after it.
Now if you've got an image/file on your hard drive or in paint, or something like that, you gotta upload it to a photo hosting site. I personally use "Photobucket", but there are other ones like Tinypic, Imageshack and Flickr. So once you've registered an account on one of those sites, upload your image to it (they'll have an option where it says "Upload"), and get the link to the url of the pic. Copy it here, then it's easy sailing. Copy it here and type [ img= before the link and put the final bracket at the end.
Bobs your uncle! Hope that makes sense. Sorry if it was confusing, I'm shithouse at explaining stuff.
Here's some of my favourites.