I don't know if this is intentional, but whenever someone puts an image with a transparent part for photo contests, the transparent part always gets turned either black (most of the time) or get some weird multi-colour combination. This is quite annoying, as I have to ask people to re-send the original image.
Could you send an example source images with this weird multi-colour background to support@pollunit.com?
We convert all images into the JPG format. Therefore transparencies are currently not possible. But we can put something else in the background instead of black. White, grey or tiles like in Photo Shop. Would one of these backgrounds be better?