Another Approach to Clarify Preterite and Imperfect

Every year when my students ask "When do you use CAMINÓ instead of CAMINABA?" I feel overwhelmed (and a bit scared!) because I know that I don't have a way out, and I have to explain when to use preterite and imperfect.  Teaching through Comprehensible Input implies avoiding grammar rules and memorization, as we are trying to use words in context knowing that eventually students will "get it".
This year, in my attempt to find a quick solution that will satisfy the need of an answer without making it a long lecture and a list of rules to memorize, I used the picture below from "Where's Waldo?"

I explained that the decision to use preterite or imperfect is based on the intention and the perspective that we have in our messages.  I had them look at the picture on my SMART board, and look at all the different situations in it.  For example, let's look at the bottom of the picture, left corner.  If my intention and the main event is to say that a woman took a picture of her husband (preterite= what "happened") while three women were flirting  (imperfect=what "was happening") with a muscular guy, I say "la mujer tomó una foto de su esposo mientras tres mujeres coqueteaban con un hombre musculoso".
The imperfect is the "background" of the picture that we want to paint for our listener/reader, so that s/he has a bigger idea of what happened.  If my intention, or the "main" event that I want to tell is that three women flirted with a guy, and a woman was taking pictures of her husband is the background (not the main event of my message), I switch perspective:  "Las tres mujeres coquetearon con el hombre mientras la mujer tomaba una foto de su esposo." I had some volunteers do this in English.  They looked at one specific situation and chose it as their "main event, or reason for the message".  Then, tell the class what event was that, and describe what was happening around it. There were a lot of expressions such as "ahhh" and that made me happy!
I don't know if this helps you, but certainly my students said that this exercise clarified this subject for them. Do you have any ideas to facilitate comprehension of this topic?