Unfortunately, there is no way to add a picture on a specific inventory item, but you are able to add a picture of the compartment as a whole. You may upload a picture of a compartment as a whole through the Compartments section.
Here's how to:
Under Inventory, click the Compartments section.
Then, click Create Compartment Layout.
Within this layout, you may attach the picture of your inventory item. Once you are done, you may click Create.
If you want to add a picture to a compartment that has already been created, simply click the desired compartment and click the Edit button as highlighted below. Then, follow the same step above to attach a picture and click Save.
Please see the following links for more information regarding inventory: