Underneath most question types you can add a button 'Additional image'. This button allows the respondent to upload an image as extra information. E.g.: When you ask how they liked the food they can upload an image to show what their plate looked like. When you ask which was there favorite part of the event they can upload a picture to show why they liked it that much.
The uploading of an image can't be made mandatory. If you want to have this mandatory you're looking for the question type Image upload.
Tip: You can change the text of 'Additional image' in Labels and messages.