Found the answer:
1. Open the desired image file using the Preview application.
2. In the menu bar at the top of the screen, select Tools, then Adjust Size...
3. A window should appear. On the right side of the window, click the box that states pixels and select your desired measurement.
4. There should be three text fields along the left side of the window labeled Width, Height, and Resolution. Type in your desired values.
5. When you're finished, select OK.
6. Finally, remember to save the file before closing it.