How do I upload just one image for both eBook and print? I need it to be high quality for print, while avoiding large files that incur eBook digital delivery fees.

Your best option when you’re working on print and ebook simultaneously is to make sure everything is up to standard for print, then use the automatic theme option to compress your images for your ebook export. To do so, go to Appearance > Theme Options > Ebook Options and select Reduce image size and quality, and your next ebook export will contain compressed versions of your images, reducing the file size. (But your print image sizes will remain the same.)