Christopher Taylor posted this image on his Catalogue of Organisms blog, and it has raised a little controversey about the details. The intent is that the relative size of each organism in the image is representative of the number of species in that group. So the large fly represents the huge number of insect species. Towards that intent, I believe the image succeeds, but I have read some disagreement about the specific numbers used to develop the image.
Two very similar images with some differences are also available. One from the University of Sydney:
In case you're wondering where the mammals are, we're represented by the reindeer cowering underneath the mushroom.
And another on from Cornell University: (this link wasn't working for me)
No matter which is exactly correct (and there's no way to tell), you get the point how small number of species of mammals are compared to the others.
Thanks Kevin, for sending in the link!