You can add an image to an object to distinguish it visually from other objects of the same type. For example, you have modeled your organization as a business actor, but you want to distinguish it from other business actors by adding your company logo to the shape. This can be achieved by adding an image to the object.
Business actor with company logo added as image
Initially, the image is placed in the upper left corner of the object. You can drag it to another location within the shape if needed by selecting the image and dragging it to the right position. Do not move the image outside of the object shape. Then it will get detached from the object and become a separate graphic shape on the diagram or view.
If necessary, you can also resize the image to make it fit the object.
Technically it is possible to add an image to a relation, but the image will not be attached to the relation, like it does with an object. The image will be a separate graphic shape on the diagram or view. If you would move the relation, the image will not move with it. You will need to separately move the image, or select it together with the relation before moving them.