From Miss Mertens' photostream on flickr.
Helmtrud's dress shows the cleaner lines of the era around 1910, but she wears a moderate-sized hat, blouson bodice, and the skirt is close rather than tight so this is probably from before 1910.
Keywords: Helmtrud of Bavaria, Princess, Wittelsbach family, German, straight coiffure, floral hat, blouse, high neckline, collar, buttons, over-bodice, lapels, long full sleeves, cuffs, blouson bodice, close skirt, necklace