SpringerImages provides access to over 1.5 million images for biomedicine, life sciences, & medicine.
Search by subject and limit by image type to find what you need. Copy images or register to create sets and download high quality ppt or pdf images.
AccessMedicine search for images, audio and video in popular textbooks. Citation information is saved along with the image for easy citing.
Flickr Creative Commons Search for images under each type of Creative Commons licenses. Types are explained on the main page.
Google Images use this advanced search page to limit to images "labeled for reuse." It would be wise to double check when you view the image.
Always provide a citation for your image!
Consider the purpose of the presentation.
Should the presentation stand on its own or is it just there to lend support to your talk.
Focus on what you want to communicate.
Don't overdesign, less is more.
Use graphics to communicate, we've all seen enough clip art for our lifetimes.
Make sure your text is readable from the back row. If you can't read your text in slide sorter view, it is probably too small.
Embrace empty space.
View gridlines on the slide to arrange and balance texts and images.
Use transitions or animations very carefully. Unless very well done, they distract from your content.
Check out these sources and tools for better presentations: