Tracking leads from banners gif or swf on external websites

It would be nice if you could do this to get the impressions, but you don't need this to track clicks if the links are coming to your website and/or Marketo landing pages.  Just add one or more querystring parameters to the ad with unique value(s) for things you want to track (e.g. campaign, content, ad size, etc.).  You can use the Google utm parameters.  When the visitor hits your page, you can have a triggered smart campaign add them to a static list, update their Lead Source field value, etc.  The only thing you can't do is add anonymous visitors to a banner ad program Program, but once they become known they can be added after the fact if you want.

GIFs & SWF files by nature can not have javascript embedded in them, no. so you can't add the marketo munchkin code to a graphic itself. Instead as Elliot has already suggested, you complete this action by providing a tracked URL that goes to a page with Marketo munchkin code on it. Be sure you have a campaign written for 'listening'  and properly noting who comes from which ad. 

If you aren't familiar with using hidden fields, here's some further reading.

Building off of what Elliot and Adam have suggested, I've used the URL parameters to do this by leveraging a "Most Recent Lead Source" field.

Field Setup
I've created that field in SF for the pure purpose of capturing the last touch of a lead. I then added it to the form as a hidden field and set it so it gets it's value from the URL parameter. Make sure the field can be overwritten (in Marketo Admin settings).

Campaign Setup
On your landing page, build the URL so the the mrls parameter uses your unique campaign name. Anytime the landing page is competed with the URL parameter, BannerAddXYZ will populate in the Most Recent Lead Source data field.
http://info.abc.com/ContentLandingPage.html?mrls=BannerAddXYZ

Listening Campaign
Now do what Eliot suggested and set up a listening campaign.
If XYZ form is completed and Most Recent lead Source = BannerAddXYZ, then....
--Add to list
--Send Alert
--etc