In Garry's Mod 13, all silkicons are included in the game. They can be found in materials/icon16/*.png
Also included are flag icons. They can be found in materials/flags16/*.png
The flag icon filenames follow the ISO 3166-1 alpha-2 country codes and include all flags
Make sure to include icon16/ before the .png as shown below
This line of code makes a DermaMenu option have a silkicon logo on its left side.
Here is an example of using a silkicon in a DHTML panel.
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.