1516x1008 has a rather odd aspect ratio - 1.50397:1. Try to resize it manually to an aspect ratio of 1.25:1 or 1.3333:1, or better yet, make it to one of the dimensions _DRU_has mentioned.
From the FAQs page: "The server will accept images at the following pixel widths and heights: 800x600, 1024x768, 1152x864, 1280x1024, 1600x1200. We chose these sizes because they are the most commonly supported resolutions on current video card and monitors. Please upload the largest image size that you have. The server will automatically make the smaller sizes for you."
You'll notice that 1280x1024 is 5:4 ratio - some desktops sizes do follow different ratios. I'm fairly sure you can upload 1280x960 too, although it doesn't have it on the list (again, standard 4:3 ratio).
"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." Bertrand Russell
^^^^^^
Can someone please explain what this means, to me? I tried to upload a picture and that was my response.