This is actually an extremely old post from 2013. If you have a transparent PNG you can just upload it and it will maintain its transparency.
If you have any further issues with this, can you please email our support an example image and describe your issue to support [at] ribbet.com. It may be that your original image doesn’t actually have transparency (not all PNGs do).
Or, let us know if you have any further questions about this. Thanks!