This guide is for a single weapon's icon. It's located in materials\vgui\hud. You need to download a file in the workshop and unpack it to see how it works.
icon_pistol
icon_pistol_magnum

icon_pumpshotgun
icon_shotgun_chrome

icon_autoshotgun
icon_shotgun_spas

icon_smg
icon_smg_silenced
icon_smg_mp5

icon_rifle
icon_rifle_ak47
icon_rifle_desert
icon_rifle_sg552
icon_rifle_m60

icon_hunting_rifle
icon_sniper_military
icon_sniper_scout
icon_sniper_awp

icon_grenade_launcher

icon_crowbar
icon_machete
icon_fireaxe
icon_frying_pan
icon_baseball_bat
icon_cricket_bat
icon_tonfa
icon_katana
icon_electric_guitar
icon_knife
icon_chainsaw
icon_golfclub
icon_pitchfork
icon_shovel