so i would first prepare the file in png format, with the correct transparency. then create a material in archicad, turn the transparency layer on. then assign that material to a wall element to only one side of the wall. the other materials of the wall should be an empty material. alternatively, you could use a picture frame for the same method, but i think it would be easier with the wall, due to the limited material settings in the frame gdl object.
you wont get a perfect decal, but it will do the trick fast. here's a video showing how to do all this:
https://www.loom.com/share/3395000938cc47c1b51ce438a2f75048