How to create a transparent image with CSS?

The CSS3 property for transparency is opacity. IE9, Firefox, Chrome, Opera, and Safari use the property opacity for transparency. The opacity property can take a value from 0.0 - 1.0. A setting of opacity: 1 would make the element completely opaque (i.e. 0% transparent), whereas opacity: 0 would make the element completely transparent (i.e. 100% transparent).A lower value makes the element more transparent.