Following on from Justin: the objective doesn't relate to Marketo, really.
The question is whether you have a service that can generate the static image you want based on a simple HTTP GET from an <IMG> tag (i.e. non-JavaScript-assisted, since emails can't run scripts).
In general such concepts are widespread (we use LiquidPixels, for example, for dynamic product images) and surely there's a GIS/mapping service that can do this. Even if there's not a direct <IMG> server available, with so many mapping APIs out there, you can build your own (compositing dynamic ➴ widgets with a preset map of the US is easy for a developer).
Alternately, you could call a webhook to pre-generate images and populate a URL field on the lead (this would be notably faster at open time, esp. if you're building your own image server and the server isn't equipped for massive real-time traffic).. Though still basically the same idea on the back end.
