Two things about adding images to a powerpoint (or other) presentation:
First of all, you need to cite the sources of all your images. In a presentation, just add a caption with a citation under each of your images.
Secondly, images with a ratio of 4:3 fit well in powerpoints. You can actually use Google Advanced Search to limit your image search to a set ratio.
Google Images can be a great place to go if you want to get some idea of what sorts of images are out there.
Use an Advanced Google Image Search to filter by size, resolution or topic, and to find images that are free to use, share and even modify.
You may also use images in the public domain. Check Wikimedia Commons images for images that are free to use and share.
You will also find images from the British Library and Folger Library on the online resources tab of this guide .
Remember that you still need to attribute the image to its proper source!
PowerPoint presentations need not be boring or simplistic. PowerPoint offers a surprisingly robust set of tools for academic work.
You may add recorded narration to your slideshow, customize the look and feel of your slides and transitions between slides, embed video, audio and images, and more - all to make the content of your presentations reflect the complexity of your subject.
The video tutorial below introduces some of PowerPoints features.