It's a mix of stock photos, scenes generated by Manifold itself, and contributions by friends. If I recall correctly, the image of a flower on the 9th had also been modified to shift hue for more blue. The image of storm clouds over a field yesterday I'm guessing was stock, but it might have been contributed. One of my favorites, the closeup of a sunflower with a bee that appeared last month, was contributed by a friend.
If anybody wants to contribute an image, it has to be in the public domain. It should be primarily horizontal aspect, and big enough that after reduction by any cropping it can be at least 2000 x 700, with a not-too-busy central region that allows the text to stand out. Images get staged about six months in advance so if an image is contributed it could be a while before it appears. Almost all US government images (NASA, Defense, etc.) are in the public domain.
For the last two or three years every day has been a new image. That's built up a collection of over a thousand unique images. There's been some push back that using each image just once means the good ones are missed by those who don't visit the site every day. So this year, the plan is to have a new image every day taken from the past collection, but avoiding the weaker ones. New scenes generated in Manifold or contributed will also appear.