The Select Image dialog lets you manage the images that you use in your model diagrams, and to insert them into your models as object symbols, diagram backgrounds, etc.
From the Symbol Format dialog Custom Shape tab, click the Enable Custom Shape check box, and then click the Browse button next to the Shape type property (see Symbol Format Properties).
From the Symbol Format dialog Fill tab, click the Modify button in the Fill Effects group box (see Symbol Format Properties).
Right-click a diagram symbol and select Change Image.