Imitation -- Not Copying -- Is The Highest Form of Flattery!
While I do not care for people infringing on people's copyright by using their images, text or other material without permission -- doing so without crediting the artist is worse. Copying the work and then claiming it as your own work is an even greater crime and truly theft!
Copying an image and not attributing it to the artist of the work in a sense might be -- by omission -- implying perhaps that you have rights to it or that you are the creator -- depending on how known the artist is. By not connecting the work to the creator whether artist or author, you also are stripping away the identity of that artist and making it nearly impossible for anyone to find other work by that artist.
In the past I have discovered some of my writing and artwork where someone has claimed it as their own. While it is flattering, it also gives me a bit of a creepy feeling.
Imitation might be the highest form of flattery, but imitation implies that someone has done some work. It is doing something in your style or in the manner that you are doing it -- not taking your work as their own.
With some forms of work the dividing line between style and complete content might not be so concrete as in others, but I think most can tell if they look at the work behind the original.
I am lucky that I am a bit of a detective. I can search out an artist who created an image I found on the Internet where no direct indication of their identity has been left with the image. Perhaps I am just a bit compulsive in my searching and willing to spend some time searching through a few hundred images. I am also a bit skilled in finding the proper keywords for a search to find what I am looking for. Sometimes I will search for things associated with my target to find better keywords and clues. Perhaps I am a bit obsessive with that too.
I can often succeed in my searches and make my findings known where possible to undo some of that damage if I can.
So if for some reason you are creating a collection of images or "whatever" remember to also record who created the item and where you found it... when is sometimes important too. Perhaps my hanging around Museum and Archaeology types is showing?