Use the built in PREVIEW application, it can resize and crop, though it can not do batch processing as far as I know.
This is the icon for it, found in the Applications folder.
Also have a look at the following links, as the information presented there might be helpful in your future endeavours into Mac OS X and could clear up initial confusion and may even prevent harm to your system or your files.
I have set JustLooking as my default image viewer for jpeg's and have them open in Slide-Show mode by default. This way I can easily flick through all the image files in a directory...