Go as a partner to the partner login and from there to "My Profile" (menu item on the right. There select "Partner WAWI" and then the menu item "My Business" at the top.
On the left you will find several menu items, one of them is called "Upload your own picture". Click on it if you want to exchange a picture. You can delete the old one and upload a new one. Please note that the photo must be in JPG format.
This photo can be seen by customers in your store. So it is worth taking a photo that flatters you and presents you in a friendly way.