You could use an agent script with an image and get the image source from a variable. Populate the variable based on the image you want displayed. If the scripting tools don't do exactly what you want, you can always embed a web page of your own design and implement anything you want in that page.
Thanks Tim, in our case we've a preview campaign so there isn't any strategy. We'll use a variable that will open a URL from a custom field.
BR and many thanks.