The British Museum has an excellent collection of 374 drawings by Rembrandt and his school. They have developed an online research catalog around this collection, which includes essays and images. In some cases, there are both front and back images of a drawing. You can click on image thumbnails to get larger images that are 750 pixels wide. These should be sufficient for PowerPoint. However, larger images are available for personal use at no charge if you are willing to create an account, make a request (quick and easy), and wait up to 48 hours to get the image. This can be done by clicking the “use digital image” link/icon below each image (it looks something like a floating piece of paper).
Note: Many of these images are available in ARTstor. However, the majority of the ARTstor images were scanned from slides that were shot from books. The images on the British Museum’s site are of much better quality.