The URL is the internet address that is assigned to your image. I clicked on your profile photo, then, in the upper right-hand corner, near the rating of the photo, you'll see 'original' 'small' 'large', etc. If you click on one of those, you'll get a page with just your image on it. Copy the address line at the top of your screen, and paste it into where they want the URL. For your profile pic, it (the URL) is:

http://images.summitpost.org/original/46531.jpg

If I copy that URL, and then highlight it, and click on the "Img" button on the reply screen, it will place the picture into the reply, like this:



(if you quote this reply, you'll see how I did it)

Hope this helps