iResize is a small graphics editing tool for the Mac. It was designed to let you quickly resize your photos. It might sound like you would find tons and tons of applications that do this. But they are not that easy to find, and the ones that you do find are often not really good. iResize manages to be free, simple and it works really well.
The application consists of three separate windows. The one on the left is the image list. Here you add all your images and they get listed, so you can double click on them to open them in the editor. The editor opens them in full size and it lets you rotate images one by one. On the right are the image resize options. There are two ways in which you can resize an image, you can either increase or decrease its size in pixels or you can compress the images and save them to JPEG. Making an image smaller in resolution will effectively reduce its size, but if you don't want to change its size and want to have more control over how much smaller your files will be, compression is the way to go. There is even a compression preview utility that lets you see how your images will look and what their size will be after processing.