The detail window supports also the use of a picture. Only one picture per detail window is supported.
The imagecan be set with
As you can see from the sample the image position needs to be given in pixels relative to the left upper corner of the window.
The image is just referenced by name. The actual image data needs to be send to the frontend component upon application start.
Please check ABAP Development: HowTo add own images (e.g. for pins and icons) for further details on this.