How to change Screen Capture Naming Convention

RE "Screen Shots" , not screen "Grab"

MAC OS X Snow Leopard 10.6.8

Shift-Command-3 takes a screen shot. Shift-Command-4 allows you to select a part of the screen you want captured.

When I do this a png file is saved on the desktop and it is named something like this: " Screen shot 2011-07-24 at 2.39.24.png"

On a collegue's MacBook Pro 15" unibody running Mac OS X Leopard 10.5.8 (pre Snow Leopard) the file is saved as "Picture 1.png".

Some people prefer the date & time stamp, others do not.

SO, how does one change the automatic naming convention (assuming there is one), from the datstamp to something else, like Picture n.png (n being the next available integer).

BTW, I do prefrer the PNG format for its transparency, quality and small file size compared with jpg.