There are four steps for using images in your timeline project:
1. Find an image.
2. Determine if the image URL can be used in Timeline JS. (The URL needs to end in .jpg, .gif or .png.)
3. Determine if the image is out of copyright and can be used freely.
4. Cite the image.
Of these steps, the first one admittedly is the most fun! Here's a great place to start:
Now you need to figure out the rights status of the images you've chosen from this collection. Typically, that information will be available in the "About" section of the website. In this case, here's the information you're looking for, with the relevant passages highlighted:
"This image(s) is a digital reproduction of works from the collections of the Bibliothèque nationale de France that are no longer protected by intellectual property rights. The use of these contents for commercial purposes is subject to payment and covered by a license. Commercial use includes the resale of the contents in the form of prepared products or the supply of services. For commercial use, contact: firstname.lastname@example.org. The use of these contents for non-commercial purposes is free of charge, subject to compliance with applicable French legislation and notably the inclusion of the source’s statement."
This means that you can use the images on your timeline, with proper citation, because you are creating the timeline for educational, non-profit reasons.