On macOS, Preview.app is highly underrated for all it can do. It’s my go-to app for a lot of things — one of them being fast image resizing.
Let’s say you have an image that is too big for where you want to use it. Preview can quickly cut it down.
- Open the image in Preview.
- Click Tools → Adjust Size.
- On this dialog box, you can resize the image to whatever size you want. I recommend keeping Scale proportionally checked so that as you adjust the width, it automatically adjusts the height so it doesn’t distort the image.
- Click OK, and then save your new image.