I'm trying to make an asset id finder but it's not detecting that it's not an asset?

Ok, so I have this asset ID finder that's supposed to find the asset id of this decal. But, it stops at one, how do I fix this so it works? I hope you can help!

script.Parent.MouseButton1Click:connect(function()
local difference = 1
local decalID = 1150480718

while true do
	wait() 
	if "http://www.roblox.com/asset/?id="..decalID - difference == nil then
    difference = difference + 1
	else
		print(difference)
script.Parent.ImageLabel.Image = "http://www.roblox.com/asset/?id="..(decalID - difference)
return print(script.Parent.ImageLabel.Image)
end
end


end)