I have a tagged PDF that has a one remaining accessibility problem: some background images were never tagged as background images. Is there a way to do this in Acrobat Pro DC without also tagging the foreground content (which is properly tagged)? If I use the "Touch up reading order" tool, it seems I can only select a rectangle, then everything in that rectangle ends up getting tagged as background. If I use the "Find Element" dialog, then press "Tag Element", there is no option to tag as background or artifact.
You seem to have experience tagging, so maybe this is off-base for your task, but when using the touch up reading order (TURO) tool, you should see the outlines of the objects on a page. In my example, the object that needs to be tagged as background can be selected by clicking on its number (in this case 1) and then clicking the background button in the TURO window.
********Edited based on comment
If the figure was never tagged as anything before, have you tried using the Content Panel instead of the TURO? You'll see the untagged content (elements outside of the containers--ignore "Annotations") here.
In this example, my untagged background image consists of two paths. I can right click on each and choose Create Artifact. Because these are both in a row, I can also select both together before creating an artifact, but never do that if there is anything in between the two.