How do I make an ImageLabel on a ScreenGui change?

I think you were using the link for the asset instead of the texture link.. If you put just the numbers from one of the links "rbxassetid://SOME NUMBERS HERE" into the imagebutton's image property, then roblox will convert the assetid to an image link that you can use in your scripts.

Here's a video showing how to do that: https://streamable.com/0adok

Anyway, I did that for all four of your faces, and put them into a table:

local faces = {
	sad = "http://www.roblox.com/asset/?id=4620142770",
	neutral = "http://www.roblox.com/asset/?id=4620142572",
	mad = "http://www.roblox.com/asset/?id=4620142387",
	happy = "http://www.roblox.com/asset/?id=4620142149",
}

Since I put the links in a table, you can now access an image by saying faces.sad or faces.happy etc

You don't have to have them in a table. You can have them as normal variables like you did in your question, but having them in a table allows us to loop through the faces.

It was good that you named your variables, but having them as actual words instead of one letter variables helps you know what things are like in the example above.

To test that all the faces work, I looped through them when the player clicked the button:

--this is a table containing all your reaction images
--you can access an image by saying faces.sad, faces.neutral, etc.
local faces = {
	sad = "http://www.roblox.com/asset/?id=4620142770",
	neutral = "http://www.roblox.com/asset/?id=4620142572",
	mad = "http://www.roblox.com/asset/?id=4620142387",
	happy = "http://www.roblox.com/asset/?id=4620142149",
}

--this is your image button to click
--i have this script directly inside my gui. 
--change this to be the location of your image button. 
local imageButton = script.Parent.ImageButton 

--function that runs when your image button is clicked
imageButton.MouseButton1Click:Connect(function()
	
	--this loop displays all the faces inside our faces table
	for expression, image in pairs(faces)do
		imageButton.Image = image
		wait(.5)
	end
end)

if you wanted to make the image be the happy face when you click it you can do the following:

--this is a table containing all your reaction images
--you can access an image by saying faces.sad, faces.neutral, etc.
local faces = {
	sad = "http://www.roblox.com/asset/?id=4620142770",
	neutral = "http://www.roblox.com/asset/?id=4620142572",
	mad = "http://www.roblox.com/asset/?id=4620142387",
	happy = "http://www.roblox.com/asset/?id=4620142149",
}

--this is your image button to click
--i have this script directly inside my gui. 
--change this to be the location of your image button. 
local imageButton = script.Parent.ImageButton 

--function that runs when your image button is clicked
imageButton.MouseButton1Click:Connect(function()
	imageButton.Image = faces.sad
end)

let me know if you have any more questions