When phpBB was installed it was probably installed into a folder called phpBB2/3 or forum(s) that's what most people do. The files and folders inside of that folder are in the root directory of the forum.
If you gave us a link it would be easier to say more exactly.
In that folder you'll find a folder called images with other folders in it, you can create yourself another folder in there and keep your images in it, to address those images you would specify the relative path images/your_foldername/image_name.ext
Hopefully you can see that the relative part of that refers to the fact the it hasn't got a / on the front, so it starts in the forum root folder, and then includes the entire path to the image you want ?