Getting the UserID of a player by clicking them?

First, let's get start by getting the part a player is clicking on, and checking if they are a player.

LocalScript in game.StarterPlayer.StarterPlayerScripts.

local services = {
	players = game:GetService("Players"),
	user_input = game:GetService("UserInputService"),
}

local imports = {
	user_input_utility = require(script.user_input_utility),
}

local configuration = {}
-- This is the maximum amount of studs away we want to handle clicks. The most we can do is 5,000 studs with my code. The longer we go, the worse the performance is.
configuration.maximum_click_distance = 500
configuration.image_link_format = "https://web.roblox.com/Thumbs/Avatar.ashx?x=250&y=250&Format=Png&userId=%d"

local player = services.players.LocalPlayer

local function on_user_input_began(input, is_engine_handled_input)
	if is_engine_handled_input == true then
		-- This is input Roblox is handling, such as clicking in the escape menu. We want to ignore this.
		return
	end
	
	if input.UserInputType ~= Enum.UserInputType.MouseButton1 then
		-- Ignore the input. This means we won't let mobile or console players use this feature.
		return
	end

	-- We want to ignore the local player's character when trying to find what they are clicking on.
	local mouse_target_ignore_list = {
		player.Character,
	}
	local mouse_target = imports.user_input_utility.get_mouse_target(input, configuration.maximum_click_distance, mouse_target_ignore_list)
	
	if mouse_target == nil then
		return
	end

	-- We know the local player clicked on a part, but we do not know if the part is an item of the player's character.
	local mouse_target_player = services.players:GetPlayerFromCharacter(mouse_target.Parent)

	if mouse_target_player == nil then
		return
	end

	local clicked_player_image_link = string.format(configuration.image_link_format, mouse_target_player.UserId)

	-- The image link we want.
	print(clicked_player_image_link)
end

services.user_input.InputBegan:Connect(on_user_input_began)

Here's the user_input_utility ModuleScript inside the above LocalScript:

local user_input_utility = {}

local services = {    
	workspace = game:GetService("Workspace"),
}

local camera = services.workspace.Camera

function user_input_utility.get_mouse_target(input, maximum_distance, ignore_list)    
	if ignore_list == nil then
		ignore_list = {}
	end
    local unscaled_ray = camera:ScreenPointToRay(input.Position.X, input.Position.Y)    
    local scaled_direction = unscaled_ray.Direction * maximum_distance    
    local scaled_ray = Ray.new(unscaled_ray.Origin, scaled_direction)
    
    return services.workspace:FindPartOnRayWithIgnoreList(scaled_ray, ignore_list)
end

return user_input_utility