An interesting feature that the ui module should have is the option to set a region of the image to be displayed, something like coordinates x and y within the image and width and height from that point, this would allow you to join several image into a single file and then set parts of that larger image to be displayed.
This feature should be implemented in Picture and ListItem, TabItem, ComboItem etc...
And it could be available through a method, for example:
local picture = ui.Picture(window, "image.png")
--picture:setRegion(x, y, w, h)
--picture:region(x, y, w, h)
--picture:setViewPort(x, y, w, h)
The same could be available for loadicon, perhaps through optional parameters x, y, width and height.