Goto your forum admin area and goto "Main Forum Config" Change the images location to You must be logged in to see this link. You have to have all images for your forum in this folder or the missing ones will be broken on your forum.
If you already have images you want to use you would ensure that the name of each image corresponds with the name of the image in use on your forum. You can find out the exact name of the image by right clicking on it and selecting properties.
I'll make a zip file of all images and make it available for download to make it easier.
You cannot at the moment. Believe it or not you cannot download anything from our servers at the moment apart from html and its a pain to figure out how to allow zip files.