Hi Cauda,
the program is not designed to let you do that, but you can achieve it using a workaround. Just search in your hard disk the images used as billboard and replace them. They are located in the following folder:
C:\Users\Public\Documents\Lands\Plant Textures
Have in mind that for species that show seasonal changes there will be several billboards to replace.
And please note that this is a workaround. If other users find it useful we can consider to add some user interface to do it in a controled way.