In order to add any HTML content to a survey, including but not limited to an image, you will want to add a question to the survey of type "Caption". To do this:
  1. Go to Content > Surveys
  2. Next to the survey click "Edit"
  3. Navigate to the "Include Questions" step
  4. Click on "Add Questions"
  5. For question type choose "Caption"
  6. In the "Question Text" area, you can add any HTML content of your choosing. When you are done click "Finish"
  7. Change the order of the questions if necessary to place the HTML code in the proper spot on the survey.