I don't know anything about the Mac, but they come with great graphic programs that have they capability to resize, Mac's are known to be user friendly, what happens if you double click on a picture, does a program open it? You can resize it there. Anyhow, until someone can give you better help with the Mac, here are a couple of online resizing places. Just follow the instructions, usually 6x8 inches is a nice resize.
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