Another prime example of irony. You started the discussion about this and now you’re blaming us for going into it? The debate here is the credit of login images and whether they can be called art or not. And here you are asking us to take that discussion elsewhere.
And yes, it is art. If we’d put the original, unedited, image on the login page and place an user’s name below it, your point would be valid. But this here is a parody of the original work. Every change, no matter how minimal, makes the original creation different than what it was before.
Can parodies of something be called art? They surely can. The image or picture they put the text on is no longer the art, it became a source for the new artform.
Let’s take the following image for example:

Well then, what are the sources here?
- My Little Pony: Friendship is Magic by Hasbro.
- Comic Sans by Vincent Connare.
- Possibly even the “KHAN!” phrase from Star Trek II.
By your logic of an image macro. The above image macro should have the sub-title “Artwork by Hasbro”, the comic sans just being the font the creator chose.
But now look at it from the other way. Why not have it say “Artwork by Vincent Connare”? Surely his font can be seen as a form of art. The image macro above was just one of many image macros that chose to use his font. Instead of a different thing placed on the image, here they choose to give the original artwork a different background.
So is the artwork from the image macro by Hasbro or Vincent Connare? No, it certainly is not. The person who created that image macro used both MLP:FiM and Comic Sans as sources for his new creation. Which makes the creator of that image macro the artist of it and therefore the result should be given the title “Artwork by whoever”.
Tl;Dr Parodies like image macros are their own form of art. The stuff they were created with are sources the artist chose to use to make the combined result. Therefore “Artwork by X” is completely justified.