I am sorry that I do not have other editing software or know how to use a Mac, so this is all done in Adobe Photoshop and or Elements for a PC.
Open your image from the camera, select the Marquee tool. Make sure the settings at the top are set the way they need to be.
The Style should be set to "Fixed Size", width at "800 px" and height at "600 px". Remember we are doing an 800 x 600 crop, the size can be anything you want.