Why the native Grabber application in OS-X can’t save in someting other than a TIFF file format is beyond me, and a whole lot of other people besides. Over at Lifehacker, the fans have it all figured out. Shameless quote:
Check out TInkertool/Onyx. They’re both really nice tools for macs, but the nice thing is they include an option to change the Grab filetype.
Also, if you want commands to take screenshots, in PNG, look no further then below:
Command(Apple)-Shift-3 – Whole desktop/menubar/etc.
Command(Apple)-Shift-4 – You drag and define the space it takes the picture.
Command(Apple)-Shift-4-Space – It allows you to take a picture of anything, and just that thing, whether it be a window or something like that.
It saves the files on the desktop as Picture 1.png, Picture 2.png, etc.
Hope this helps.