Hi Luke,
Here are some steps I've screenshotted to help you:
It is the direct link you need to copy (You'll need to click the Insert Image from the forum text editor and paste it in between the img & /img tags), you could use the IMG code from photobucket which will save you having to click the "Insert Image" icon in the forum editor.
Looking at what you have posted previously you may have selected the "Email & IM" option which doesn't work.
To quickly test it works, open a new tab in your browser and paste the "Direct Link" in to the address bar and press Enter (Like I have shown in the last screenshot). If you have copied the correct thing your image will load.
Thanks!
Dan