Free Tiiu Pix is a site maintained by a retired teacher, Tiiu Roiser. The purpose of the site is to provide students with hundreds of free, high resolution images that they can use in their presentations. In addition to the free images Ms. Roiser provides some good advice to students about designing and delivering better PowerPoint presentations.
Free Tiiu Pix has a good system of categories and labels that will help students quickly find images that are suitable for their presentations. There are sixteen main categories on the site and dozens of subcategories. The two largest categories are the plants and animals categories.
Applications for Education
Finding free, high resolution images for a presentation can sometimes consume as much of a student's time as drafting the outline of a presentation. Having a resource like Free Tiiu Pix bookmarked could save students a lot of time. If you have a "resources" section on your class blog or website consider putting Free Tiiu Pix on it.