A simple way to create collages to use for backgrounds on a website or social media platform such as Twitter is to take web screenshots of photo collections. The sunflowers are from one of my photo albums on Facebook. I took a web screenshot of the album page with Aviary (an amazing extension on Google Chrome) and saved it on my hard drive.
The train collage was taken from a snapshot of a Flickr set from the Museum of Transport in St. Louis. It’s part of an Andrography collection – pictures taken with an Android Smartphone. You can upload collage images and set them to repeat in a background.