Just click the 'browse' button at the bottom of the reply box.This will bring up the different places in your computer where you can store your pictures.
Click on the heading where your pictures are stored.This will bring up another window.Select the picture that you want to post and hit the 'open' button in that window.
This will open it into the file at DG where it will post in the link. Hit "preview" just like you do when posting to a thread and check your picture to make sure it's the one you want. If so,hit 'submit'.
If you've messed up and it's not the right picture,just hit your 'back' button and repeat the first steps.
I don't think you can put in pics from anywhere besides your hard drive right now. But if your photobucket page is open to the public, you can always just include a link to it instead of posting the actual picture. Probably won't get quite as many people to look at it that way, but if someone's really interested to see what you have then they will take the time to visit the link.