Its a homage when they give visible credit to the originator and should also pay a royalty (just like sampling in music)
Otherwise its theft which you can also call swipe
I also consider it an homage when the source is so well known everybody knows it. Mind you, the original artist should still be credited, but if not, it's usually obvious that the current artist expects that everyone will get the reference.
Swipes are harder to find, since they rely on people not realizing there is a source cover being used, so by nature the original covers are less well known.
Out of those three, I want to read Ka-Zar in space. That has to be full of exceptional nonsense.
I've read it, and it most certainly is full of nonsense. Last issue of the series, and they pretty much threw any and every thing they could onto the pages.
"As a youngin' I used to share the colt 45 on the street corners with my friends. I'm not proud." - icctrombone
"If you are strong, be a protector. If you are smart, be a humble influencer. Strength and intelligence can be weapons, so do not wield them against the weak. That makes you a bully. Be bigger than that." - Chris Pratt
The fact that there is a Highway to Hell but only a Stairway to Heaven should tell us all we need to know about expected future traffic.
Whereas this Jim Lee cover from earlier in 2015 is intended as an homage, as on the middle right side of the cover it's noted "after Kane" (the Gil Kane Avengers #145 cover shown in Crimebuster's post). The Superman cover shown in the OP also has an "after Steranko" notation.
EDIT: So, for this cover and the Superman cover, the later-day artists are acknowledging their debt to the inspiration-covers' artists, in case readers don't recognize or are unaware of the source covers.
To me it's a homage if it's meant to be noticed. Nobody is "swiping" the cover to Action #1. When you see it you know what it's referencing, like an inside joke.
It's a swipe to me when it's using an interior panel of an obscure page and is being essentially traced without reference. Pretty much if it's on the cover it's okay if it's on the inside it's a swipe.