To add an image at the top of a survey, use the following HTML text in the question text box when creating/editing the topic:
<img width=75% height=75% src=INSERT THE URL>
You can adjust the size of the image by changing the image width and height percentage.
For example, <img with=100% heigh=100% src=www.mindmixer.com>
The image will then appear above the survey question(s).
Please note, the question text box has a maximum limit of 250 characters, including the HTML text.