IMO STK's icon used the the launcher does not look all that good, and when small kinda just looks like a red blob. My proposal is to use gui/main_race.png for 128x128 and smaller as it's simplicity means it looks like something more than a blob, and use gui/logo.png for all bigger sizes (e.g. Android app icon, gnome-shell launcher) as it is big enough to be properly visible.
I made a version of main_race.png from the svg in the media repo that looks a bit better as a small icon.
Here's how it looks:
16:
32:
48:
64:
128:
And the full logo:
128:
256:
512:
Alternatively logo_slim could be used for the small logo:
A real-world comparison:
My icon:
logo_slim: