Here are some examples of graphics on a page with background color.
A picture saved as a jpeg. The jpeg format cannot make an area of the picture transparent.(pic1.jpg)
A picture exported as a GIF with transparency and the Matte Color set to white.(pic2.gif)
The picture exported as a GIF with transparency and the Matte Color set to None. The edges are aliased- they look jagged. (pic3.gif)
An anti-aliased version of the transparent GIF is created when you set the Matte Color to match the background color of the web page. (pic4.gif)