I have lots of high-resolution images inserted into the Keynote which makes the file very large. Image masking in Keynote is very convenient for me. But I only want a small corner of the images shown in the slides, and I don't need the cropped part anymore. Reducing file size under the File menu loses image quality and the cropped images are still there. Is there any way to delete the cropped part to reduce file size without losing image resolution/quality?