I had to figure it out for myself, but here's what I do to post a pic. Register and stuff at photobucket, when you log on there a button/area to add pic, click "browse" and then I go to My Pictures and find what I want.
Once it's in at photobucket I highlight the URL, copy (ctrl+C), then go back to DS, and when posting click on the mountain thingy picture and paste in the URL.
It sounds like a lot of steps but it moves pretty quick.