Over at phylopic.org there is a large and growing collection of silhouette images of all manner of organisms – everything from Emus to Staphylococcus. The images are free (both in cost, and to use), are available in vector (svg) and raster (png) formats at a range of resolutions, and can be searched by common name, scientific name and (perhaps most powerfully) phylogenetically.
[EDIT: as two commenters have pointed out, not all phylopic images are totally free of all restrictions on use or reuse: some require attribution, or are only free for non-commercial use. It’s best to check before using an image, either directly at the phylopic webpage, or by using the phylopic API]
Phylopic images are useful wherever it is necessary to illustrate exactly which taxon a graphical element pertains to, as pictures always speak louder than words.
Below I provide an example of using phylopic images in R graphics. I include some simple code to automatically resize and position a phylopic png within an R plot. The code is designed to preserve the original png’s aspect ratio, and to place the image at a given location within the plot.
I should also point readers to Scott Chamberlain‘s R package fylopic, which provides the ability to make use of the phylopic API from within R, including the ability to search for and download silhouettes programatically.
If you find phylopic useful, I’m sure they would appreciate you providing them with silhouettes of your study species. More information on how to submit your images can be found here.