Since you've already loaded the image up to photobucket, the easiest thing is to just put image tags around the photobucket url. You can do that two ways:
1) type the url, then highlight it and click on the little photo icon in the message menu above the input field. It looks like a picture of a mountain with the sun above it.
2) type the url, then type "[img]" without the quotes immediately before it and "[/img]" again without the quotes immediately after it, like: