I have also tried to understand the situation and have created the following example.
local ui = require("ui")
local win = ui.Window("sample", 220, 230)
local gbx = ui.Groupbox(win, "LuaRT", 20, 15)
local pic = ui.Picture(gbx, "__tests\\LuaRT.png")
function pic:onClick()
ui.info("You have clicked on the Picture !")
end
ui.run(win)
It works perfectly. Maybe you could post the section of your source code.