Website Poster
Usage
Requires Imagemagick. Can be improved (a lot).
Download or clone collage to your hard drive, open a terminal and cd to your the folder where collage is. Then execute:
$ ./collage
You may want to change the depth and extensions variable in the script to customise the use of this program.
Demo
URL: https://openclipart.org/search/?query=abstract
License
Website Poster is under GNU/GPLv3 License.