Try the below:

# Dropzone Action Info
# Name: Screenshots
# Description: Take a screenshot, add it to ~/Screenshots and add to Drop Bar
# Creator: Gareth Evans
# URL: http://yoursite.com
# Events: Clicked
# KeyModifiers: Command, Option, Control, Shift
# SkipConfig: No
# RunsSandboxed: No
# Version: 1.0
# MinDropzoneVersion: 3.0

def clicked
  # Create the Screenshots directory if not already present
  system("mkdir ~/Screenshots")
  
  # Make a filename with the current time
  formatted_time = Time.now.strftime("%F at %I.%M.%S %p")
  filename = "Screen Shot " + formatted_time + ".png"
  file_path = File.expand_path('~') + "/Screenshots/" + filename
  
  # Capture screenshot
  system("screencapture -i \"" + file_path + "\"")
  
  # Add to Drop Bar
  $dz.add_dropbar([file_path])
  
  # Notification
  $dz.finish("Screenshot successful")
  $dz.url(false)
end

Also, could I recommend you fork the https://github.com/aptonic/dropzone3-actions repository, add your action and send me a pull request? That will make your action show up at https://aptonic.com/actions/untested automatically which will allow others to install it.

This will also allow us to collaborate on improvements more easily

I've managed to create a simple screenshot plugin using the 'screencapture' command in OS X,

It's early days, but thought I'd share what's going on so far:

# Dropzone Action Info
# Name: Screenshots
# Description: Take a screenshot, add it to ~/Screenshots and add to Drop Bar
# Creator: Gareth Evans
# URL: http://yoursite.com
# Events: Clicked
# KeyModifiers: Command, Option, Control, Shift
# SkipConfig: No
# RunsSandboxed: No
# Version: 1.0
# MinDropzoneVersion: 3.0

def clicked
# Create the Screenshots directory if not already present
  system("mkdir ~/Screenshots")
# Capture screenshot.png to the directory
  system("screencapture -i ~/Screenshots/screenshot.png")
  file_paths = [File.expand_path('~') + "/Screenshots/screenshot.png"] 
  $dz.add_dropbar(file_paths)
# Notification
  $dz.finish("Screenshot successful")
  $dz.url(false)
end

My biggest hurdle is when you take a second screenshot, it looks like there are two files in the drop bar, but both icons point to the same screenshot.png,

Does anybody know if there is a dz.remove command in the API so I can put it above the dz.add command, ensuring there is only ever one icon in the Drop Bar.

Also, if there's an easier way to approach this I'm all ears