-- The variables for the button.
local players = game:GetService("Players")
local plr = players.LocalPlayer
-- The function for when the button is clicked.
local function onClick()
local id = plr.UserId
local thumbType = Enum.ThumbnailType.AvatarThumbnail
local thumbSize = Enum.ThumbnailSize.Size420x420
local content, isReady = players:GetUserThumbnailAsync(id, thumbType, thumbSize)
--sep--
local imgLabel = game.StarterGui.ScreenGui.Frame.ImageLabel
imgLabel.Image = content
end
-- Call the function above when the button is clicked.
script.Parent.MouseButton1Click:Connect(onClick)
I put this script inside the button. No errors, yet didn't work.

2 comments
Is the ImageLabel visible? If not, you need to set it after the line 'imgLabel.Image = content'.
use PlayerGui not StarterGui