I had a little extra time, so I made some simple solid color images in Microsoft Paint and saved them as 24-bit BMPs.
I opened them up in Hex Edit, and the kids were able to figure out where the metadata stopped and the pixels began. We manually changed a string of pixels to black, then exported it out and looked at it in Paint to see the changes.
I think/hope this helped them realize how close the encoding in a BMP file is to what they did in 2.4 (Encoding Color Images)