Sourcing images for a project such as this can sometimes be difficult. The Visual Resources Association (VRA) has put together this extensive list of fair use image sources - the list was published in February 2017, so it is up-to-date for the time being, but things may change, so be sure to double check before using an image.
My approach was a bit different, as I knew the I had to go through the Centennial catalogue and identify all the images mentioned in the text before searching for their current location. This was
While an Omeka project you complete for Penn is almost surely Fair Use, as it is educational and not for profit, it's still worthwhile to go through the Fair Use Checklist created by Columbia to make sure. This is something you may want to bookmark if you plan on doing other web based projects outside of an educational setting.