These are the sample input files in $PWD for the below examples:

1.png 2.png 3.png 4.png 5.png 6.png 7.png 8.png 9.png 10.png 11.png 12.png 13.png 14.png 15.png 16.png

Here are some sample executions and outputs:

    python collage.py          \
      --background '#00000000' \
      --width 2000             \
      --height 1000            \
      --density 23             \
      --noise 100              \
      --border 170             \
      --shadow-size 3          \
      --  *.png

    python collage.py        \
      --width 2000           \
      --height 1000          \
      --density 23           \
      --noise 100            \
      --background '#8f1c1c' \
      --border 170           \
      --shadow-size 4        \
      --  *.png