I'm really not sure about hacking with one of those things... depending on where you live, can't you just buy a second-hand desktop or laptop for next to nothing? If you're hacking the NES as you said, you can use something over 10-15 years old and FCEUX will still run perfectly fine, then you can use all the programs you need. I'm sure Chromebooks have their uses, but I can't see hacking being one of them.
Regarding how graphics look in a hex editor, sure, you could do it, if you read documentation in how graphics are stored. But seriously, your time is worth more than hacking graphics that way.