[Tipzntrix]

I would guess they pull from the wiki commons and then use typical JPG or PNG algorithm libraries to stretch or compress the image. There's probably some AI to retrieve images that don't need to be stretched much so that it looks natural.

[jlogsdon]

From their images list:

> The provided images are for layout purposes and each image we use for this project is released under the creative commons license (CC BY-SA). For more information visit http://creativecommons.org/licenses/ So, if you want to use the images for more than just layouts, you have to double check the license and ask the author. For this we will link to each photographer's flickr page. Furthermore, we assume no liability.

[jldteixeira]

Almost any picture can be used, the algorithm just needs to stretch and crop to maintain the correct image size ratio.