It depends what size you have it set to in the settings on Photobucket.
If you click the big orange upload button at the top, you will be redirected to another page. On that page, is a white box surrounded by a dashed line. At the top right hand corner of this box, is a little cog for your settings, click on it.
Another little box will drop down, make sure the 800x600 option is ticked. Click save and then upload your picture.
Once it has uploaded, click on the photo, and it will come up with a bigger one.
To the right is a box with a load of links. We want the bottom one, with the word IMG to the left. Clicking the link, the box will turn yellow and say "copied". Now simply paste the link into a reply box on the forum, and, tad-ah!
You have to post the message before the photo appears, it stays as a link whilst you write your post!
Hope this helps,
Steven