IOS Developing:Tab Bar Item's Image Display a Blue Square

The bar item images I prepared are about 35*35 and I use the Sketch to export the [1x & 2x] size images; (I can't insert any images for instances at here because of my zero reputation//Sorry about this!)

Then it displays a blue square :<

the function and title of Item is OK

BUT just the icons are wrong

Could someone tells me how to fix it! Appreciate!!!

via Chebli Mohamed