Some transparent images, especially monochromatic ones are prone to be "invisible" on a background of the same color. 

Consider using checker background commonly used when rendering images with transparency effects for preview purposes.

To consider:

• Do it with all transparent images? Rendering them on a background without the checkers actually looks very nice e.g. for icons, if their color is different from the background and can actually be made less discernable with checkers.
• What about thumbnails? The problem is there too. 
• Could we render white transparent images black instead, circumventing the issue? This could however wrongly suggest that the image actually is black. (would need dev research)