The transparency of image can be done using opacity property available in css.
When mouse is over your image it is know as hover over image which is also property in css.
Example to Enable opacity is:
.image{
opacity: 0.5;
}
Opacity range from 0 to 1. You can have any value in between these range.
Example Images:
You can also refer to link...https://www.edureka.co/search/web+development