You can do this by following the steps below:
1) Open your Media Library in the session (top bar) and select/add a image file from your computer by clicking the black plus button. It will then be added to your "Media Library" and will be converted for you. You can open it in the content viewer via double click.
2) Once the image displays, you should click on the "Show Whiteboard" button (small button at the bottom right of the content viewer) to activate it. You will see the whiteboard appear on top of the image and you are good to go.
But remember, the image that a "Moderator" uploads can be seen by all the participants present in the session. And to use the whiteboard you have to give the
participants these permissions or convert them to a moderator. So basically if the image is edited by one participant it will be shared on every participants screen.