When you have added an XML galley file in the production phase, click the blue triangle in front of it and then edit. In the opened modal window you will find a grid where you can upload artwork files. It works the same with HTML files. That is a fairly hidden in OJS.
When you upload for example image1.jpg there and have a reference to the same filename in the XML galley file, both the embedGalley plugin and the lensGalley plugin should show the image.