This is the one thing I can easily do on a PC and not a Mac. Any suggestions on how to easily do this?
You can do that in preview. Also, I am sure if you search www.macupdate.com for simple free image editors you will find a plethora of them.
@jackm‘s suggestion will definitely get you some simple image editors, but if you want to get something a little more advanced (on par with Photoshop) check out GIMP for Mac.
Use this widget.
have you tried resizing on iphoto?
The inbuilt picture viewing program Preview can do this. By the way, are you trying to crop an image to 100×100 or resize it to 100×100?