Note: If you're getting redownload error message after you've already redownloaded the script, just give it time and try again in 5 minutes!
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 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.