EDIT: I've been trying to upload my art, but no matter what I try, I doesn't show up. I tried changing it to a jpg, I tried giving it numbers..... NOTHING WORKS! EVEN THOUGH I CAN UPLOAD SCREENSHOTS OF SPACE ENGINE!!!!!
Check the size of the file if it does not exceed 600 KB.
Otherwise if you have a Google account or other, "host" your image, put into sharing user offering the link and made copy paste on your post. If it is an image, it is [img]url[/img].