The image must be amazing. Do not post "owned" pictures or anything else that you find merely funny. It must be something that you look at and say, "Woooow" or whatever your expletive of choice is. Think St' Peter's Cathedral/Everest/Grand Canyon. That type of amazing.
Host the images on a site and post them here, or link to the image on another site. We don't want to waste server space by attaching them all.
Credit the original source if possible. If you for some reason don't know the original artist/photographer/whatever post all the information you do know along with the image.
Have some sense about image size. Given that awesome images may need to be large, do not worry about resizing the images to be smaller before posting, but if you are posting a 20 inch wide panorama, please just give us a link.
Avoid images from games. I know there is a lot of cool stuff in games, and I know we are all gamers, but the focus of this thread should be on photographs, paintings, etc. not screenshots.
Give us background information on the image. We want to hear good stories, and amazing things usually come with amazing stories.