Why won't my asset images load?

--Local Script
local cp = game:GetService("ContentProvider")
local toLoad = {}
for i,v in pairs(game.ReplicatedStorage.ItemList:GetChildren()) do
    for _,item in pairs(v:GetChildren()) do
        for index,value in pairs(item:GetChildren()) do
            if value.ImageId.Value ~= 0 then
                table.insert(toLoad, "http://www.roblox.com/asset/?id=" .. value.ImageId.Value)
            end
        end
    end
end
for i,v in next,toLoad do
    cp:Preload(v)
end

--Later in the script...
itemFrameClone.ImageLabel.ImageButton.Image = "http://www.roblox.com/asset/?id=" .. item.ImageId.Value

I tried pre-loading images, but it made no difference, unless I am doing it wrong. There is always an error that says "failed to load texture format" and the image never shows on the ImageButton.

Help appreciated, thanks!