Slight update to Chapter 7 images

Thanks to John N. Rayner for pointing out that figure 7-41 wasn’t online. It turns out this is because the script that I wrote to convert the images refused to convert that figure for me because it was compressed with an algorithm that ImageMagick doesn’t support for legal reasons. That compression was used because the publishing team tweaked that image at some point, and I simply didn’t notice.

The problem has now been corrected, and I apologise for any inconvenience.