Using PNG instead of GIF ??

[Whiterook]

Due to the inherent properties of the gif (the jaggies), I've changed my icons (location; images/icons/icon1-14) to png's using my image editor.

The problem is when I upload them to the site, the extension gif remains.

Question:
Is there a template/script/ some where regulating the extension for those icons?
When I view them on the site, and click to save loacation, the icons still read .gif.

I'd like to be able to use the png's for the clarity of the icons. I could of course use a background color for the icons, but you still get the slight border lines showing.

Thanks for any help you can offer.....
Regards,
WR