If we're looking at some sort of objective standard for unethical swipes, we need to first decide what is ethical. Here are some suggestions.

If you're working on , say,  a Batman comic, to swipe poses, compositions, panel sequences etc in a flashback to a specific event (in order to evoke the original story and artwork) is OK.

If you're doing a pastiche or parody (like what Will Elder did so remarkably well), any copied or evocative imagery may be permissible as long as it remains clear that it's intent is precisely evocative. (this may cover homages as well, though the intent here is more praise or tribute than parody)

If you're copying your own work,  whether artwork or photos staged by you or any other form of art to which you own the copyright (or where you work with the approval of the copyright holder), that is ok. This covers among other things the use of likenesses and designs for licensed work (movie and TV adaptations).

If the artwork or photos being copied are in the public domain then there are no legal implications. For recognizable works (such as the Mona Lisa) the work should be evocative of the original. For generic works (such as drawings or photos of buildings,vehicles or clothing) any public domain work may be used as photo/art reference.

For artwork and photos still in copyright, these should be avoided when specific (as in works of art or currently in commercial use) but may be used when generic (depicting only real life or actual persons without staging or compositional properties that clearly are parts of the photographer or artist's personal expression) This one is a bit iffy. Basically sources like these are more in need of change or "reinterpretation" and care should be taken to not copy them too closely in terms of composition and camera angles.

For art or photos still in copyright, but that have a historical significance, there are fair use allowances that should enter into it. While, say, photos of the 2 towers burning on 9/11 2001 are doubtlessly still in copyright, the historical significance of the event would make an exact copy of such art accpetable. This also applies to a lot of other news stories and historical records as well as the depiction of the likenesses of famous people and politicians in specific noteworthy circumstances.

Any literal tracing (whether lightboxed, photoshopped or artographed) of another artist's copyrighted commercial work (whether drawing or photo) without their permission is not ok.  

Does that work?

Let's go a little further here.

What about Jack Kirby's collages? I think these appeared as early as his FF stuff, in the Galactus trilogy. The pictures where not credited and clearly they are photos and not drawings. The only apparent creative effort was in the arrangement and the physical cutting and pasting.

This is what now: shortcut, laziness, swiping, rip-off, homage?

I wonder whether Goodman figured he didn't owe Kirby his normal page rate when Jack delivered this batch to the office.