The wiki is a collaborative tool. It is very interesting and effective if you want people working together on some project. With little information you've given I'm not sure that the wiki is the perfect tool for your project.
I will maybe better suggest using the forum for this. Each student is creating a reply to your first message asking them to put a picture of their achievement
If you really want to use the wiki I would suggest that all the students are sharing the same wiki, and then they are building the page altogether showing each other their result with the help of picture
By the way, to answer your question, I don't see why you wouldn't be able to add more than one picture in your winki
Could you be more specific about the procedure are you following?