I think people tend to miss the basic important stuff. I would:
1)Define a pixel, and how an image is made up of pixels. Then I would elaborate on that and explain how resolution and data are finite, explain up sampling, down sampling, explain how DPI actually works...
2)Discuss RGB values, and bit depth.
3)Introduce color spaces and basic color management. Explain how an RGB value is only a number and doesn't become a color until it is passed through multiple devices that attempt to use a common understanding of what said value is (ICC).
4)Discuss destructive editing.
then maybe move on to actual interface and tools like layers and brushes.