There’s an extension Location Images by Emily Short that does this automatically – you just need to declare that The room-illustration of Some Room is Figure of SomeRoom.
It attaches to carry out looking rather than before going so that the look command will restore the location image, for example if you have illustrations for examining objects that could sometimes replace it with something else. (And also because before going will still trigger even if the move is prevented by another rule, which can cause unexpected behaviour.)
Incidentally, if you have a look at the Kinds tab you can see that the default value for a figure name is Figure of cover (which is the cover image for your story, which can’t actually be displayed unless the game is released anyway). Assuming that you don’t want to use the cover image as a room illustration at some point (which is usually a reasonably safe assumption), you can exploit this to remove the need for a dedicated empty image:
A room has a figure name called the illustration.
Definition: a room is illustrated if its illustration is not the figure of cover.