As the reel images are so small (about 11.5mm x 10mm) you can use images from pretty much any source, including your Facebook/Instagram/Flickr/Snapchat/iPhoto libraries. However, the larger and better quality your photos are, the better quality your reels will look when magnified in the viewer.
The images are landscape orientated square-ish rectangle. You should bear this in mind when selecting your images. While it is technically possible to use upright images or wide horizontal images on a reel they don’t look great, as they view much smaller.
If you are a whizz with Photoshop (or other image editing programs) you can supply your images to the correct dimensions of 1240px wide x 1125px high @ 300dpi. You might find this template handy for image preparation: Retro-View template