Well if you look at RPG::Cache in the help file under def load_bitmap, you will see :
def self.load_bitmap(folder_name, filename, hue = 0)
path = folder_name + filename
if not @cache.include?(path) or @cache[path].disposed?
if filename != ""
@cache[path] = Bitmap.new(path)
So basically, it would be something like:
self.contents = Bitmap.new('Graphics/<folder_name>/<image_name_with_no_extensions>')
However, don't use this method. The RPG::Cache holds a cache of loaded bitmaps. Istead of reloading the same bitmap over and over again, it is better to use the RPG::Cache to load bitmaps (think of it as a memory. don't learn 2 + 2 again, memorize it)
self.contents = RPG::Cache.picture('filename')
something like that.