The problem comes from resizing your original piece down to 100x100. You'll have to upload the original 200x200 piece as the 'detail file' and crop out a 100x100 or lower preview.

Click the 'Edit' button up there to change the files.

Also you should post a link to the original base you used.

FYI, you can't downsize pixel art as of course pixels would be lost! Any pixel resizing (up) should use 'nearest neighbor' settings.

No, I didn't use any special tools...the site I drew this on said I only used I think 8 or 9 colors.

This is a very cute unicorn. :)

I checked the color count and there seems to be 848. Palette management is important in pixel art. There's no specific number of colors something has to have, but the more you have the less necessary each one is. Try deciding on a set palette before starting, maybe 10-20 colors, and just use those.

Did you use some tools that have options such as opacity and flow adjustments or automatic anti-aliasing? Things like those will cause the color count to rise very high because the computer is choosing many new colors for you and applying those to individual pixels on its own. Instead, try using just a pencil tool with a size of one pixel and maybe a fill bucket or a rectangle tool. You don't have to click each pixel one by one, especially if it's a large area that's going to be one color, but it's important to have personal control over the work at a pixel level in the hands of the artist.

Thank you very much! :3

Welcome to PJ mate!