Hi. I don't think this picture is going to work. Pixeljoint prefers uploads to be true pixel art at original resolution: that means relatively low color counts, no computer calculated gradients/color blends and every pixel is actually one pixel.

This picture is 1398 * 1020 with 31799 colors (many of which are indistinguishable to the human eye). If this is based on pixel art that you made, it may be better to upload the original pixel art  at it's original resolution instead.

Hope that helps.