This workflow does the following:

• Asks for your current theme directory
• Creates a temporary directory
• Opens http://localhost:3000
• Asks you to choose a window to capture
• Saves the fullsize screenshot
• Crops it to 256x256
• Runs /sw/bin/convert on the full size image to reduce it in size and quality, plus turn it into a jpg.
• Drops the resulting two files in your theme directory
• Deletes the temporary directory