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Cura

Thumbnail

You have to add a Post Processing Script to add "G-code thumbnails" in Cura. To add these scripts, click on Extensions > Post Processing > Modify Gcode. Then click on Add a script, chose Create Thumbnail and fill in the size 32x32. Add again the same script with the size 400x400.
Important is, that the size is square! If you don't use a square resolution, Cura will squeeze the thumbnail.

Direct G-code upload

Install the OctoPrint-Connection plugin in Cura’s Marketplace. In Cura go to Settings > Printer > Manage Printers and find the Button Connect OctoPrint. Add your printer and type anything in the API Key field.

Best practise G-codes

In Cura, there is a straightforward solution to add custom G-codes. Pedro Lamas has developed an excellent script that you can use as post-processing to add everything necessary.
Follow the guide here to use this script: https://github.com/pedrolamas/klipper-preprocessor​
If you think Pedro's script is excellent, feel free and pay Pedro a coffee: https://ko-fi.com/pedrolamas​