Welcome! It makes me happy to know children are being exposed to the Glowforge in their art class. Engraving photos is not one of the more straightforward tasks a Glowforge can perform, and I would suggest not starting with photos on cardboard. I think it would be a frustrating experience.
You can start with tracing some simple drawings to get started. Here is a file you can use that has simple geometric items for cutting. Cut Shapes Template I think you will find it extremely helpful. Right click on the image and choose Save As SVG when prompted. Then upload the file to your Glowforge library.
There are lots of people here in the forum that will be happy to help you and your students get maximum use of your Glowforge. Many here had no laser experience when we got the Glowforge, and we appreciate that it can be hard to get started. Once you work through the 1st three prints and a few of the tutorials, come back to the forum with questions and someone will help you get to the next step.