In Garry's Mod 13, all silkicons are included in the game. They can be found in materials/icon16/*.png
Example of using silkicons
This line of code makes a DermaMenu option have a silkicon logo on its left side.
MENU:AddOption("Say hi", function() RunConsoleCommand("say", "hi") end ):SetImage( "icon16/user.png" )
You no longer can use surface.GetTextureID and surface.SetTexture with silkicons, because they are now .png files. Now, to draw them, you must use Material and surface.SetMaterial. Since Material is slower than surface.GetTextureID, it is not recommended to call it in a drawing hook, which are ran every frame, as that will cause lag. Instead, save the material as a variable outside of the draw hook.