To add an image to your Project Dashboard, first you need to upload it to Lumi.
See how to upload media.
Go to the Project Library to see all media in the project.
You can refine your search by typing the name of the image and/or clicking "Images" underneath the search bar to search through all images in the project.
When you have found the image you wish to add, click on the image and then select "Add to Dashboard" from this window.
You can choose whether or not to put a header on your item i.e. a title for this picture on the Dashboard. Click the dropdown menu for "Header", type the header and click the checkbox "Show Header".
When you're finished, hit "Create" to add to the Dashboard.