How to use avatar decals?

EDIT I answered too soon!

There's a few more things we need to change.

GetPlayerFromCharacter is a method of the Players service, not hit.Parent (that's the character). UserId is a property of the player, NOT the character. You pass in the string "Humanoid" whereas we need to pass in the parent of the object (say a leg or an arm) which is the character.

Here is the wiki article on player UserId's.

local players = game:GetService("Players")
function onTouched(hit)
   local player = players:GetPlayerFromCharacter(hit.Parent) 
    if player then -- This will only run if our player variable exists.
        local userid = player.UserId
        game.Workspace.LocalUserId.Value = userid
        wait(0.1)
        game.Workspace.join.SurfaceGui.ImageLabel.Image =('http://www.roblox.com/Thumbs/Avatar.ashx?x=420&y=420&Format=Png&userId='..userid)
    end
end

script.Parent.Touched:connect(onTouched)

In order to append or concatenate strings in lua you want to use the .. operator.

Here's how you do that with your code.

game.Workspace.join.SurfaceGui.ImageLabel.Image =('http://www.roblox.com/Thumbs/Avatar.ashx?x=420&y=420&Format=Png&userId='..userid)

Some examples.

print("my ".."name is ".."kools") -- my name is kools
local var = 3
print("# is "..var) -- # is 3