Turns out there is no filter that asks wether or not an imagefield (or filefield) has a value. I came across this tip on Drupal.org. The recommendation was to use a relationship, and then choose "Require this relationship" to act as a filter. Pretty cool.
So assuming your imagefield is just called "Image", here is the walkthrough:
This article is also available in French. Displaying hierarchical content is a common request of database-driven web sites. Drupal gives you great flexibility in doing this without writing a single line of code, using a couple of battle-tested modules. As an example, let's say you're building an automotive web site. Your site is going to have a list of automotive brands and the vehicles they build. You'd like to only have to enter each brand's information into the database once, then simply refer to that brand whenever you add one of their vehicles to the site. In this example, I have 2 brands, Chevy and Jeep, and each brand has 3 vehicles. Drupal's powerful CCK module allows you to define different content types for your web site. The Node Reference module, included with the rest of CCK, allows you to set up hierarchical relationships between content types.