Visual aids can be taken from many sources. You can find pictures on the internet, in magazines (of any age or language); you can use personal photographs, pictures you have drawn yourself. It’s also a good idea that you have put out a request on this website. I’ve copied your question into one of the general forums to help you reach a larger audience.
If you want to try to introduce critical thinking, choose pictures that have an unusual aspect or deal with a subject that incites discussion and thinking (e.g. not just a picture of a person but a picture of somebody in a group with an unhappy facial expression). People and faces make for interesting subjects to talk about. You can show two or three people together and ask students to speculate about their relationships, what they think of each other, etc. You can also ask them to write out the conversation that is going on between the people. Again, unusual situations work well for this.
Don’t just assume that students will be interested in pictures; you need to introduce them in a way that will interest your students. To awaken interest in a picture, you could flash the picture for one second and ask them to say what they’ve seen. Do this by holding the picture with one hand on either side with the picture facing towards you; turn it around quickly with a flick of your wrist and quickly back again. The first time they might just see a blur of colours; but as you flash the picture more times, the students will see more and more. When you think they can give a sufficient description, show them the picture. By this time, they will be really interested in looking at it. You can turn the picture more quickly if it is mounted on a piece of card.
You could also think about which topics are being studied at school. Try to find out which period of history your students are currently studying, which geographical subjects they are covering, etc. Then find pictures that fit in with those topics. You can even take pictures from students’ history or geography books. If they already have some knowledge about the subject, they will have more to say when discussing the pictures.
However, having said this, I think you need to be realistic about the critical thinking element. You say your students are of a basic level, so they will be limited in what they can express. Also, adolescents don’t like to be different to their peers. They usually express agreement, which doesn’t always fit in with critical thinking.
If you would like some more ideas, please write in again.