Hello,
For some of the products you uploaded images in the gallery – http://prntscr.com/pk49qd , together with swap effect (Theme Options > WooCommerce (Shop) > Shop > Products style >
Image Hover Effect), it gives such effect.
Add this code in Theme Options > Custom CSS > Global custom CSS:
.content-product .image-swap {
background-color: transparent;
}
Please note that it is not recommended to add in the gallery the same image as a featured one.
Regards