If an image isn't linked to a product, you can directly add it and also change the order of the images.
Why an image didn't link
You can also deliberately unlink images.
Managing unlinked images
Navigate to the product that's missing one or more media files. For example, from the left navigation menu, select Products and then select a product image.
Click the ellipsis menu icon under the display image and select Manage Media/Images.
In the Manage Media/Images window, the Manage tab displays the images currently linked to the product. You can hover over an image row to reorder or remove the image.
To link images, navigate to the Unlinked Images tab.
From the Unlinked Images tab, review the unlinked images. Narrow your search with the Search field in the upper-right corner or widen your search to display images linked to another product with the Show all images, not just unlinked option.
Select the checkbox above the image(s) you want linked to the product, and then select Add.
Return to the Manage tab for a final review and reorder the images (if needed). In addition to hovering over a row to display the reordering icons, you can also click and drag the hamburger icons to reorder images.
When you're done, select Save to confirm your changes.