Since librarians like to share with one another, Oberlin Science Librarian Alison Ricker recently made the Art Library aware of two great resources for digital images of insects and plants.
Aluka is a resource for international scholarly research about Africa, and one of the research areas is plants. The Pretoria African Plan Slide Gallery, available online at Aluka, has over 3300 images to view.
Odonata Central is another OC but is a great resource for photographs of Odonata, which are damselflies and dragonflies. It has over 280,000 images of these insects available.