Reduce number of jpeg images used

[k-dominik]

jpeg images are used a lot in this material. I might be living in my small bubble, but I personally, would never suggest or encourage anyone to convert their images to jpeg due to lossy compression. Still a lot of images used here are jpegs. I think that conveys a wrong message. But again, if you guys such strong use of jpegs without any downsides on your sides, then close this :)

[tobyhodges]

Like #39, I'm going to leave this one open and try to collect feedback from beta pilot Instructors to help us determine if this is still something we want to address.