If I opened that Image via Paint.NET (another Paint Program that you can download) and if I look closer, this is the result:
If you look closely at the grid at the background, that means it has a 100% transparency (means anything that is in the grid gets invisible in the Image/Graphic once it is saved) which tecnically in SMBX isn't really supported with transparency like this...
Here is what is should normally look by default that is 100% fully functional in SMBX:
Black = Seen/Visible
White = Unseen/Invisible