The "fair use" doctrine allows limited reproduction of copyrighted works. The relevant portion of the copyright statue provides that the "fair use" of a copyrighted work, including reproduction "for purposes such as criticism, news reporting, teaching, scholarship, or research" is not an infringement of copyright. The law lists the following factors as the ones to be evaluated in determining whether a particular use of a copyrighted work is a permitted "fair use," rather than an infringement of the copyright:

• the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;

• the nature of the copyrighted work;

• the amount and substantiality of the portion used in relation to the copyrighted work as a whole, and

• the effect of the use upon the potential market for or value of the copyrighted work.

Although all of these factors will be considered, the last factor is the most important in determining whether a particular use is "fair." Where a work is available for purchase or license from the copyright owner in the medium or format desired, copying of all or a significant portion of the work in lieu of purchasing or licensing a sufficient number of "authorized" copies would be presumptively unfair. Where only a small portion of a work is to be copied and the work would not be used if purchase or licensing of a sufficient number of authorized copies were required, the intended use is more likely to be found to be fair. 

1)     The purpose of these walls (wallpaper: a computer screen background) is solely for my own enjoyment, as a form of entertainment, thus as a hobby.  There is not nor ever was any commercial intent of any form;  in fact, the argument can be made that they were an expense – software, website, etc.  They are meant to serve as a form of critical review emphasizing the visual quality or attributes that I found personally appealing, and making an artistic comment relative to those characteristics.

2)     Typically the copyrighted material is a multimedia presentation of a nominal 120 minutes story line involving character development and interaction typically through dialogue and actions.  Wallpapers are static and carry no audio content.

3)     One screen cap (capture: the image of a single frame of film, 1: 30 frames per second times 60 seconds per minute times 120 minutes = 216,000;  1/ 216,000 = 4.6x10^-6) is approximately 0.0005 percent of the visual movie.  Assuming that the movie also includes dialogue, not to mention both music and sound elements, it would seem safe to say that a cap is actually on the order of a tenth of that value.  An additional adjustment should be made for the manipulations typically applied to the cap to make a wall  including such elements as text, compositing, effects, and overall layout.  This commonly would reduce the impact by at least another order of magnitude.

4)     The most common argument: walls actually serve as a form of publicity or advertising has a certain validity.  Financial gain is neither expected nor desired.  The entire act constitutes an attempt at homage – rendering an honor to, or recognition of the artistic achievement of the originator.  It would be very difficult to imagine that a film copyright owner suffered loss from my humble efforts.