Hi, I just checked the docs and I can see that up to Mendix 8 there's a guide on how to make a custom preview for a self-made building block, this option is not available for Mendix 9 / Atlas 3. Is there any way to do this in Mendix 9 or has this option been removed?
Thank you for the generosity to be kind enough to share it with everyone.
After some more digging I found the solution, for anyone wondering how:
Once you've made your building block you can open it in Studio Pro → Double click the blue bar at the top and you'll open up a settings page where you can change the preview: