This guide is intended to help you find that last image - perhaps you ran past it somewhere, and you don't want to trawl through the whole game world looking for it. It is not entirely spoiler-free, but all I will include is the name of the room, and also perhaps the name of an adjacent room (because some rooms are not easy to find).
I have arranged the images in rows and then columns. For some rooms I have also in brackets noted a room from which you can reach it - this is because some rooms are very difficult to discover, and might not even be marked on your map as an unexplored route. Another important detail is that the room name given here is the room from which the image is easiest to reach. In many cases, this is the end of the previous room, or immediately after it. Just keep in mind that if you go to the room given here looking for the image, you may need to go backwards a little.
I welcome feedback (in the form of comments), including: Any mistakes I have made Cases that aren't quite mistakes, but where a different room name would be more appropriate Cases where the extra room hint in brackets is too much of a spoiler or is unnecessary. Cases where an extra hint is necessary.