PhiloPic.org provides ready made silhouettes of animals and plants for your graphic art needs
If you want to create drawings like the one to the right using a program you already know how to use, this tutorial may help you. Powerpoint is not ideal for this sort of drawing, but it is easier than Inkscape (free) or Adobe Illustrator (very expensive), since you already know how to drive powerpoint. No new user interface to learn, just a few tricks and tweaks to get the head around.
This tutorial will take you through the key steps in generating this illustration of a newborn wallaby attached to a teat in the pouch. I will show how to generate the base by tracing the outlines from a photo, and then how to embellish this with fills and shading to get a reasonably 3-D appearance.