I need to print some images taken from Google Earth. The images are saved as JPGs. The oceans are different shades of dark blue and there are lines of latitude and longitude in white. I'm going to print these images in grayscale, but do not want to waste toner/ink by having the oceans print in shades of gray.

This is what I'd like to do: process the image so that all the ocean area is white and the lines of latitude and longitude are black.

I have an iMac, but do not have any image processing software (except iPhoto and PhotoStudio). I'd like to be able to do this without spending a fortune on high-end software. Also, it would be nice to be able to do a batch of images together in some kind of automated process.

Is this possible? Thanks.