Macs have a built in photo editor, Preview. When you open up an image (JPEG, TIFF, PNG, etc.) on a Mac it will open in Preview. As an example the photo below is 724 x 483 pixels. We will resize and crop it to be 400 x 200 pixels. Note, since Preview will automatically save your changes you may want to duplicate the image before you start. (Go to File>Duplicate)

Step 1

Start by clicking on your image to open. A basic Preview toolbar will open. To open up Preview's editing capabilities click on the Toolbox Icon. A menu bar will appear below. Click on the Image Dimensions Icon.

Step 2

Give it a width of 400 pixels. Leave the height whatever it automatically becomes. It is too tall but we will crop it in the next step.

Step 3

Clock on the Selection Tool Icon and drag it out until it reads 400 x 200. Then click on Crop.

You're done! It's now 400 x 200 and Preview will automatically save.