Smaller files work better!
Instead of having one Room_Builder_16x16.png that contains a lot of different things, splitting it up into a number of smaller files would be much easier to work with as a developer, especially when there are frequent updates.
So you could have a Room_Builder folder and that could contain:
- Walls.png
- Borders.png
- Floors.png
- Carpets.png
- Stairs.png
- etc