Use higher quality images for 3A and 3B

[allyhawkins]

👍🏻 One thing we might want to keep track of/an eye on is if the PNGs underlying panels A and B should be saved at a higher quality in the future. My PDF to PDF conversion senses are tingling. But I don't think that needs to hold things up.

Originally posted by @jaclyn-taroni in #59 (review)