The short answer – YES. Public domain pictures may be used for any purpose, even commercial. It is important to check that the photo you want to use is indeed in the public domain. The wallpaper sites are notorious for grabbing anything that looks good and claiming it to be in the public domain. A reverse image search through Google will usually turn up the original source.

Spiderman, at least the Marvel omits version is certainly NOT, public domain. The character and distinctive likeness thereof are trademarks of Marvel Comics.

The Disney versions of all those fairy tale princesses are also copyright and trademarked by Disney.

I don’t believe there are any Spider-Man cartoons in the public domain. The character first appeared in print in 1962 and so is well within copyright.

Yes, this means Roy Lichtenstein probably could have been sued successfully for copyright violation for his famous Pop Art paintings.

Spiderman, at least the Marvel omits version is certainly NOT, public domain. The character and distinctive likeness thereof are trademarks of Marvel Comics.

The Disney versions of all those fairy tale princesses are also copyright and trademarked by Disney.

I don’t believe there are any Spider-Man cartoons in the public domain. The character first appeared in print in 1962 and so is well within copyright.

This is sort of a weird and complicated subject. Similar to the example above, but to a very different outcome, the old Max Fleischer Superman cartoons have famously fallen into public domain and can be copied and used/reused any way anyone sees fit. However the Superman likeness and logo are still very much protected trademarks owned by DC Comics and/or their assignees.