Are you using Firefox? I tested just now in both Chrome and Firefox: Chrome is working fine, but Firefox has an issue if you don't strip the colour profile before uploading.
oh, you have to use a very specific image layout for this to work: if you want to use the PICO-8 font, you'll need to first make a template in Fontsy with the intended width/height/character set, then copy the characters over from your PICO-8 image and re-import it (there's important data encoded into the grid borders of the layout Fontsy uses that tells it how to parse the characters)