For Service Items, open the master data for the Service Item in Manager and select the "Picture" tab. Then click the folder icon to browse to an image:
You can modify the picture for a shelf using the same steps.
Please note: updated pictures for Shelves will appear on the start page and in the selected shop. Updated images for Service Items will only appear when you click the Service Item to view its properties:
For more information, please refer to the IT Shop Guide, Extended Master Data for a Service Item