This is a simple Garry's Mod helper function to hot-load images from a web URL for use in your rendering operations.

cl_surfaceGetURL.lua

local WebMaterials = {}
function surface.GetURL(url, w, h, time)
	if !url or !w or !h then return Material("error") end
	if WebMaterials[url] then return WebMaterials[url] end
	local WebPanel = vgui.Create( "HTML" )
	WebPanel:SetAlpha( 0 )
	WebPanel:SetSize( tonumber(w), tonumber(h) )
	WebPanel:OpenURL( url )
	WebPanel.Paint = function(self)
		if !WebMaterials[url] and self:GetHTMLMaterial() then
			WebMaterials[url] = self:GetHTMLMaterial()
			self:Remove()
		end
	end
	timer.Simple( 1 or tonumber(time), function() if IsValid(WebPanel) then WebPanel:Remove() end end ) // In case we do not render
	return Material("error")
end

Example Image:

https://cloud.githubusercontent.com/assets/2367602/24143894/673bda7a-0e7f-11e7-9d4c-53a7174672af.png

Example Code:

local function TESTsurfaceGetURL()
	local UrlMaterial = surface.GetURL("https://cloud.githubusercontent.com/assets/2367602/24143894/673bda7a-0e7f-11e7-9d4c-53a7174672af.png", 200, 200, 5)
	local Frame = vgui.Create( "DFrame" )
	Frame:SetTitle( "Test panel" )
	Frame:SetSize( 300, 300 )
	Frame:MakePopup()
	Frame.Paint = function( s, w, h )
		draw.RoundedBox( 0, 0, 0, w, h, Color( 231, 76, 60, 150 ) )
		surface.SetDrawColor(color_white)
		surface.SetMaterial( UrlMaterial )
		surface.DrawTexturedRect(0, 25, 200, 200)
	end
end

TESTsurfaceGetURL()

Result:

cache the error material
and dont use HTML for this, use an http fetch request and put the body in a file, then use Material("../data/image.png") or whatever