You already have the image that's being displayed for this invocation, so you have many options:

• Update a key in a tied hash with the file name as a key
• Append it to a logfile and let another script parse it
• Use a full-blown database
• Open a file named the same as the image, read a number from it, add one, and rewrite the file

I've listed those in my preferred order. There are many good options.