http://tc2.ca/fr/accueil.php
The critical thinking consortium has several resources for students from K-12 to develop critical thinking. I have used those resources especially in social studies. They provide lesson plans, unit plans, videos and graphic organizers. Many are available at our library or can be purchased.
Here is an example of a simple lesson complete with purpose, instruction, graphic organizer and assessment to critically analyse an image.
http://www.tc2.ca/pdf/InvestigatingImagesSample.pdf
For example, my grade 3 students were wondering what was an inference. We can look at a picture of someone wearing winters pants and a big sweater in a classroom. From looking at this we can infer that it is winter and that it is quite cold outside. Every picture can be analysed in this way.