Thanks quincy,
I'm from the UK though the site may be hosted in the U.S.
Thanks.
The copyright laws in the UK are a bit different than they are in the US but here is what
could be considered a "fair use" of another's copyrighted material (and it is important to note that fair use is an affirmative defense to a copyright infringement action and
not permission to use the material).
IF a copyrighted work is used for teaching purposes (usually in a classroom setting), that could be considered a fair use of the material. IF a copyrighted work is used for research purposes (as would be the case in a "compare-contrast" paper), that could be considered a fair use of the material. IF the copyrighted work is used in news reporting (as might be the case in a story on the artist/author), that could be considered a fair use of the material. And IF the copyrighted work is used for the purpose of commenting or criticizing the work, that could be considered a fair use of the material.
A US court will look at the effect the "infringers" use has on the market for the work, and how much of the work was used, and the type or nature of the work, and the character and purpose of use (see the "fair use" factors above).
The fact that the original artist/author is difficult to locate and/or identify does not make a use of the material a fair use. It is up to the person who intends to use the material to identify the artist/author prior to use. The fact that the work is not watermarked does not make the use a fair use. The fact that no commercial use is being made of the work does not, on its own, make the use a fair use.
With all of that said, some works are fine to use at any time. Works that are in the public domain do not require permission to use (but you must make sure that the works are, indeed, in the public domain and there are no lingering rights such as trademark rights). For all other copyrighted works, it is wise and advised to get permission from the holder of the copyrights to use the material prior to use, to avoid any risk of an infringement suit.
For a good overview of fair use factors, you can go to the following Stanford University website:
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter9/index.html
Good luck.