Hi Kirk,
Thank you so much! This was really helpful! One last thing though, after I had made the adjustments exactly as you said, I wanted to make the background pure white and retain a little more of the pencil shading around the middle of the picture. I used the selection tool to add/subtract the areas I wanted and refined it, so in theory, the parts that should be white are now ready. As you can see from the image, everything I want in white has been "checker boarded"; only now, I can't work out how to get rid of this checkerboard and actually make it white. I've tried what feels like everything, and even when I export it as a png, it comes up as a weird grey tone (see below). If you have any ideas, please let me know, I know I'm so close!
Thanks
Bethanie