There's a little button that looks like this
when you're doing replies. You click it and it asks for the url of the image, which is usually called something like "direct link" on image uploading websites such as photobucket/imageshack. Put it in then press OK
Make sure the image is not too big when you upload it, though -- that's usually another option while you're putting it on photobucket/imageshack/etc, to have it resized. I have yet to find out how to resize images on the forum, and I'm not actually sure it's possible