Quote from AGS manual:
You can also have the current inventory item displayed in its own button on the icon bar by creating a button on the GUI and setting its text to (INV) which stretches the item picture to the button size, or (INVNS) which draws the inventory item picture straight onto the button with no resizing. Finally, (INVSHR) , probably the best option, will draw it at actual size if it will fit, or shrink it if not.
-Cheers