Looks like you'd want to use imagemaps (or imagebuttons). The basic information is here: http://www.renpy.org/doc/html/screens.html
(check the sections of imagemaps and buttons). Buttons will probably give you more control over the shape of the button. You have a base image to show the room, then buttons for anything you can click on (like the sink, or a cupboard, or whatever) so that when the player clicks on them an action is run. SImply hovering over the button can cause it to change so they know it's clickable.