You'll need Python and Pip (tested with Python 2.7).

Navigate to your script directory and run:

pip install Pillow

When that's done, run

pip install requests

When everything is installed, run

python img.py dash.png 760 406 USERNAME PASSWORD

(substituting USERNAME and PASSWORD with your reddit username and password). If you have multiple reddit accounts, you can run multiple instances of the script at once with different credentials by opening more command prompts.