Upload your fan art to our Fan Art Gallery (people will see it there forever, not just while your forum post is on top). If you've done a desktop image, please use the wallpaper gallery. (Cross posting is permitted, especially if you do your first version of the artwork, and then add a border to give it appropriate wallpaper dimensions.) You can link directly to the images to insert them into your forum post, as you would with any image host.
Any artwork inspired by the Diablo gameworld is acceptable. D1, D2, D3, or anything in between.