[2] the image files are stored inside the .ArPlantX/.ArTree files. Once you insert plants in Rhino, leaf and bark Rhino materials are created and the images files are unfolded into the following folder:
C:\Users\Public\Documents\Lands\Plant Textures
They are in separate subfolders, one for each species latin name.
[1] What editor are you using? Are you creating .ArPlantX files (AccuRender nXt) or .ArTree files (AccuRender Studio)