Why can't I change the decal id on a part?

When I use my script I get this in the output: ServerScriptService.Script:5: unexpected symbol near '?'

Put when I just leave the id's (e.g. 70414028) the original face disappears and wont reappear. Like Slender's face.

When I add http:// to the id, it gives me this error: ServerScriptService.Script:5: '' expected near '/'

local face = game.Workspace.Human.Head

while true do
	wait(3)
	face.Decal.Texture = www.roblox.com/asset/?id=70414028
	wait(3)
	face.Decal.Texture = www.roblox.com/asset/?id=161970460
	wait(3)
end