I added a short video in the original post on top to show how to change the images.
In principle you just replace the diffuse map for each slide on each of the 20 columns with the material editor.
The naming is like this:
BB2 - Billboard name
C1- Column index
S1 - Slide index
Each column has 3 slides like:
C1_S1 - first slide on first column
C1_S2 - second slide on first column
C1_S3 - third slide on the first column
The same for every of the 20 columns. So if you change the image on the first slide then on every column the first slide is the same image making up the complete image on the board. This goes for every slide. The Board can show 3 images.
Hope it is clear.
I'll be uploading the Board later today to SW. Once downloaded I suggest to export the board in the "Path object" library. Do not put it in the Custom buildset library!
If you put it in the path object library, whenever you place another instance it will always add a new instance with the same original sizes.
If you put it in the architectural library, if you place in instance in the park and re-size that instance, when placing another instance it will add a new instance with the modified re-sized shape instead of the original size.
If you put it in the custom buildset library, the scenery editor will try to add it to the scene, causing all the related sub-objects within to be moved to their own zero point. So do not add it in these libraries.
Above is what I found out when testing, if someone finds any other behavior (good or bad) please let me know.