The NIH has just launched NIH BioArt Source, which provides a wide variety of images for use in scientific illustrations.
Choose from viruses, anatomical illustrations, cells, equipment proteins, molecules, shapes, and more–including brushes and swatches that can be used in illustration programs such as Adobe Illustrator or Adobe Photoshop. Users can search within categories or throughout the entire library of images and download as AI, EPS, PNG, and SVG file types. PNGs are a standard image file that can be used in any program that lets you insert images, such as Microsoft Word, Microsoft PowerPoint, or even a website or blog post.
The images provided by NIH BioArt Source are freely available and are within the public domain, and only require that you cite BioArt Source in your illustration. Currently, you must use their contact form for instructions on how to cite the use of these images in your work.
This is a great resource for those looking to do anything from using a simple image in your presentation to creating complicated illustrations.