I just discoverd the coolest thing ever! PhyloPic, a database with reusable silhouette images of organisms. Each image is available under a Creative Commons license and can be reused (for non-commercial work), some need to be attributed.

Medusozoa by Walker Pett

Fragaria, unknown creator.

Caprealla by Collin Gross, see license agreement

And here instructions by @TrevorABranch how to insert a silhouette of an animal or plant into your R plot!