Make sure you have your logo saved as a .jpeg or .png file
At the moment you can only add an image while you are in a survey. Whilst you have your survey open in the EDIT tab.
Click on the image gallery icon - its the fourth one from the right
At the top left of the next page click on Import Image
Click on select files
Find your image within your files and click OPEN
Then click Upload to Image Gallery
You will see that your image is now in the image gallery
If you hover over the image with your mouse you will see that you have options including Add as Survey Logo, click it.
You can then select the logo position, usually it is the Top Right
Select what you want then click Apply