Hopefully you already know how great Preview is. It comes built in with every Mac, it handles PDFs and images with ease, and does a great deal of basic image editing just fine, saving you the bother of opening more substantial, more expensive image editors. Here’s a tip for getting perfect square crops inside Preview.
To crop an image in Preview, make sure you’re in Select mode and just start dragging on it. A selection box will appear, which you can resize and move around until it’s just right.
If you want to crop to a perfect square, you could do it manually, relying on the pixel measurements that appear at the bottom-right corner of your selection to tell you when the selection is square.
Or you could do it the easy way: hold down Option and Shift while dragging. Now, your selection will be constrained to a square.