The graphics tab stores the social images for your app. You can upload/edit/delete images from this tab.
To add new image :
1) Click the plus button in the left corner.
2) Select the type of image you are uploading (facebook/linkedin share).
3) The dimensions for the images are given on the pop-up.
4) Click on the big plus icon next to the Graphic field.
5) Upload the image and click Save.
To edit exiting image :
1) Click on the image from the list
2) Hover over the image
3) It will display a minus button to the top left of the image
4) Click the minus button, this will delete the image
5) Then upload a new one in it's place.