Hi,

how do you make your own thumbnail ?

Thanks,

Resilient

Use Sims 4 Studio.
To create a project, make sure that stand-alone recolor (or mesh, whatever you're doing) is selected.
To make your own thumbnail, make a 64x64 pixel image in Photoshop (or whatever graphics program you want, like gimp), and decorate your own thumbnail, and upload it into Sims 4 Studio (the option right next to the place where you export/import your texture).
That's if you want your own custom one.
I just use color swatches, which you can do with Sims 4 Studio as well.