When you use the Source SDKAltering the "hl2.exe" Icon
Create your Icon (has to be 256x256p) and save it as an interlaced .png
Find a .ico converting website and make convert the .png to .ico (Gimp Is a free Image editing application that can also save as .ico)
Download and save the .ico in"\steamapps\sourcemods\MODNAME\resource" as "game.ico"
Now It provides you with all the code required to buildshould display your mod or own standalone game that's built of the same sorta mechanics. It doesn't Include the full source code (thinks like the physics and render engine would be missing so you can't steal It)Icon, which Is why you can't change useIf not then restart your own executable. YouPC
More about Source Engine Icons can be found at: https://developer.valvesoftware.com/wiki/Making_Icons_and_Integrating
These steps can change the Icon though using one of the .json files provided, there's a more In depth tutorial on It on the Source SDK forum created by Valve.also