Icon visible in the workshop

Create a new image that is 256px wide by 64px high.

Add text or apply a different layer with the photo. It’s a very small thumbnail so don’t worry about the quality as whatever you do, the quality will be low. Then export it as a PNG image to wherever you want so that you can easily find it later.

And that’s it. Icon done. It will be visible in the workshop when you buy the skin.