There are two stages to this.
If you have not yet added any pictures to ƒh then see the advice at Adding Photographs and Other Media.
Once you have some pictures added to your individuals, with a diagram displayed use Diagram>Options… and then select the Pictures tab as shown below.
Select the Preference Drop down box and set it to 1, to select the first picture attached to the individual.
Separate text boxes, pictures, shapes, and lines can also be inserted into a diagram as explained in Adding Text Boxes to a Diagram.