I assume you mean that you want to place an image in a post?
If so, first you need to put the image on a suitable hosting site, like Flikr or Photobucket, then find the URL of the image (the easiest way with most browsers is to do something like right click on the image and select "View Image" or look at Properties and Copy the URL part).
Then you just need to place that URL text (for example in GruB's post above, it's http://i37.photobucket.com/albums/e81/GruuB/DSCF0899.jpg) between some "img" tags. eg for that URL you would type in something like this:
[img]http://i37.photobucket.com/albums/e81/GruuB/DSCF0899.jpg[/img]
If you click on "Reply with quote" for a post which has an image in it, you'll see this.
If that all sounds a bit complex, it isn't half as bad as it sounds.
(I've fiddled a bit with tags in this post to make them display as you would type them in, when you submit the post, the software will in effect change this into the image).
