At the bottom of the forum thread (where you wish to post your picture) you will see two buttons one says “post quick reply” the other says “go advanced” click the go advanced button.
A box will appear, it will have somewhere to write your text and above that, a row of icons (two rows really but you can ignore the top row, as that is mainly for text size and stuff) the 14th icon along looks like a picture of mountains with a black sun behind them.
Click on that one
A box will appear, mislabelled as it is (script label) but underneath that is a box where you can either type or paste the web address of your image. Enter the location of the image and click ok. This will inset the image into the post (full size so make sure your image size is suitable. I’d say not more than 600 wide and 400 high.
Photebucket is easy, and it give you the codes so you can insert small clickable images in to your posts with easy