Version 5.1 includes the ability to create thumbnails for Klipper/Mainsail, OctoPrint, and many other applications. There's several different formats and resolutions to pick from to make sure it integrates correctly with your printer. You can find the new option on the Output tab. For Klipper/Mainsail you would just enable the "Include thumbnail images" option, choose the Klipper encoding format, and then pick what resolution you want. I found that 300x300 worked fine with our Mainsail OS, so that's a good starting point.
I'm sure this feature will be very useful, so thank you to everyone that requested it!