Find the address for the image you want, e.g. :
http://i400.photobucket.com/albums/pp87/cupradrew/100_0732.jpg
Note, that's the link direct to the image itself, not to a Photobucket page or anything else. Copy the address from the address bar of your browser.
When you're composing your post, click the Insert Image button (looks like a blob floating over two pyramids in a yellow sky), delete the start of the link that's already in the box that pops up and paste your complete link.
The code* looks like this :
Code:
[ IMG]http://i400.photobucket.com/albums/pp87/cupradrew/100_0732.jpg[ /IMG]
and the result is this :
The other option is to add the image as a link, for which the code* would look like this :
Code:
[ URL="http://i400.photobucket.com/albums/pp87/cupradrew/100_0732.jpg"]Click Here to view the image[ /URL]
and the result is this :
Click Here to view the image
* I've added some spaces into the start and finish tags in these examples so that it displays all the text for you to see. For it to work properly, remove these extra spaces.