'if' statements not working?

When you define the health variable, it records the value of character.Humanoid.Health at that instant, and doesn't change. To fix this, all you need to do is (uncomment the if statements, of course, and) put the 'health' variable within the function.

local player = script.Parent.Parent.Parent.Parent.Parent
local character = player.Character
function Hearts()
	local health = character.Humanoid.Health
    if health == 0 then
        script.Parent.Heart1.ImageTransparency = 0.5
        else
        script.Parent.Heart1.ImageTransparency = 0
    end
    if health >= 25 and health <= 49 then
        script.Parent.Heart2.ImageTransparency = 0.5
    else
        script.Parent.Heart2.ImageTransparency = 0
    end
    if health >= 50 and health <= 74 then
        script.Parent.Heart2.ImageTransparency = 0.5
    else
        script.Parent.Heart2.ImageTransparency = 0
    end
    if health >= 75 and health <= 99 then
        script.Parent.Heart2.ImageTransparency = 0.5
    else
        script.Parent.Heart2.ImageTransparency = 0
    end
end

character.Humanoid.Changed:connect(Hearts)

Hope this helped.