I am still learning about this part of it and I know some of you could do this with one hand tied behind your back.
Although you can add pictures to the BACKGROUND of a web page, when you make a group for buttons, text, (what ever) it would be nice if you could also add a background to that area too.
Though "background" may not be the right word. Maybe "Texture"?
A long time ago on the Amiga - if you ever used it - in their later versions of the O/S, you could set patterns/textures/(I can't remember the exact term/name use) and they would be "tiled" all over the area of the screen - either the window or workbench (Their name for what is called in windows the desktop). That would add a nicer look than a blank flat colour.
This addition would make some web pages a lot nicer than just having a "flat" colour around the buttons/sliders/text boxes, etc.