Students love to upload images, either for fun, or to illustrate their off-computer work. You can easily turn on and off their ability to do so in the "Student Tools" section of the Assignment creation process.

On Step 3 of 3 when creating an Assignment, you will be presented with option to turn on certain tools for your students. Select all the tools, including Images, that you'd like your students to have access to for the assignment. 

Now, when you assign the worksheet, students will have all of these tools activated on their toolbar, and they'll be able to add images to their activities.