Mozilla Connect

Whenever I paste a link in for a shortcut in the New Tab page, I upload a .png with transparency into that link (I used imgbb) and it doesn't feature the transparency for some reason. For customization purposes, I would like the transparency to exist...

We have to paste the link constantly to change the icon of the shortcuts on the Firefox New Tab page, for convenience sake, please allow us to upload an image. I don't want to spend too much time turning an image into an image link.