Seeking ambient screen recorder
You could use Automator to build a workflow to do this.  Automator can take screenshots and you need to use a shell script to upload the screenshot unless you have an FTP client which is compatible with Automator.  Bear in mind that uploading a screenshot might take longer than 20 seconds depending on screen size and your internet connection speed.  A 5-minute interval might be more appropriate.

Here is an Automator workflow which I just tested successfully to take a screenshot and upload it to an ftp server.

NB: you need to set the shell script step to ignore input from the previous step or it will fail when trying to convert the image to text.

You can save this workflow as a .workflow file and then you need to schedule it to run on whatever interval you think is appropriate.  You could do this through iCal but I think iCal would only allow you to run it daily or less frequently.  So you need something else.  You could do it by writing a launchd script or you could use a utility like lingon to create the script for you.  This way you could set it to run on whatever interval you like.