The image you uploaded here is a GIF but the one on your wiki is a PNG. How are you converting the images?
It appears this is an issue with UCP. It looks like TableWiz already linked to this thread in the UCP thread (although his post is not the first to raise this issue) so we'll see if staff have anything to say about it.
Is the image you uploaded here the same as on your wiki? I mean the exact same file. I just reuploaded your image to a different UCP wiki and had no issues with transparency.
Either the situation is changing as I inspect it or I just wasn't paying close attention. I could have swore that, when I first checked your image on your wiki, it didn't have transparency. However, on second look, it does.
The issue appears to be with the image URL that UCP uses to display the image on pages; not the image itself.