Image not changing when it is told to, just changing to a white image?

Hi, I recently made this script:

local val = true
script.Parent.MouseButton1Click:Connect(function()
    if val == true then
    script.Parent.Image = "http://www.roblox.com/asset/?id=4968347853"
    val = false
    else
    script.Parent.Image = "http://www.roblox.com/asset/?id=4968348079"  
    val = true
    end  
end)

And whenever I actually click on the Image Button, it just goes white and (the image) does not change to the asset id I want it to change to. I am also not getting any errors when I do click it. Ideas?