Note: It’s good to keep a copy of your full-sized image as they are, so it’s recommended you duplicate and work from a copy.
1. right-click to duplicate the image.
2. Open your copy in Preview and, from the Tools menu adjust size.
3. Assuming your original image was wider than 2560 pixels, adjust the image width to 2560 and make sure the resolution is set to 72 dpi.
4. Then, from the File menu, choose the option to export.
5. Set Format to JPEG. Reduce the quality setting so the file size is no larger than about 600 to 800 KB. You’ll also have to rename the file (This is because Preview will not be able to overwrite a file that’s already open)
6. That's it! You can now upload the correctly sized image to your liveBooks editor. To double check the images specs, just right-click and 'get info' to see them. Below is a picture of the image info window showing the new dimensions and file size (695 KB).