Images can't contain alpha channels or transparencies
I've found you can also just re-export the png's in Preview, but uncheck the Alpha checkbox when saving.
AFAIK png with transparency is not allowed. use jpg OR update your png (photoshop or whatever you using to create the png) and delete the transparency areas. if you work with shadows, use jpg, that will do no headaches.
it so easy...
Open image in Preview app click File -> Export and uncheck alpha