Here you see the real beauty of Sanity. There is one folder that you add all the images but by adding the PDP page to each it auto populates to the correct brand site. :)
In the upper left drop down where you choose the Brand site, you will locate the one named Products.

Once that opens, click on the Structure tab.
In the left panel first section, look for Product Showcase Images. A new panel opens to the right. Here you will see all the images for all the brands.

Here are all the attributes that need to be filled in:
Title and Subtitle attributes are optional items to be filled in.



To see a preview of the new image, you need to go to the Brand Inspiration Gallery page. Use the Presentation icon to create your preview link.




Once approved, you are ready to click the Publish button back on the image.

Image is live once published.
If it is an Outdoor or Gas log product, image will appear on all sites that have that displayed in the list.
