Each country has it's own laws, however, in the U.S., the law is quite clear.
17 U.S.C. § 107
Notwithstanding the provisions of sections 17 U.S.C. § 106 and 17
U.S.C. § 106A, the fair use of a copyrighted work, including such use
by reproduction in copies or phonorecords or by any other means
specified by that section, for purposes such as criticism, comment,
news reporting, teaching (including multiple copies for classroom
use), scholarship, or research, is not an infringement of copyright.
In determining whether the use made of a work in any particular case
is a fair use the factors to be considered shall include:
- 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.
The fact that a work is unpublished shall not itself bar a finding of
fair use if such finding is made upon consideration of all the above
factors.[3]
You have to ask yourself:
Is the work I am producing commercial? If so, then fair use does not apply.
Is the work I am producing transformative? If so then fair use does apply.
Is the work I am producing add value to the original work? If so, then fair use does apply.
If I am commenting, am I commenting on the work itself, or something else? Is the new work itself commenting on the original work, including parody, then fair use does apply.
Am I commenting on the facts or ideas within the work separate from the work? If so, then fair use does apply.
Is the work used copied? If so, then criteria becomes critical. For example:
Is the copied work published and crucial to the original work? If so, then fair use does not apply.
Is the copied work substantial? If so, fair use does not apply. It is one thing to quote a work, but taking substantial portions of the work is not allowed.
How is the work copied? If so, then this also becomes critical. For example:
Is the work covered by license, restrictions, acceptable use policies, etc.? If so, then the license, restriction, or acceptable use policy applies trumps and notion of fair use with legal restriction. This applies specifically to software, web sites, and the like where usage is restricted.
Does the copied portion of work harm the original work? If so, then fair use does not apply. There are two primary considerations: Does the copied work replace or substitute the original?, and Does the copied work harm the original beyond substitution such as licensing, marketing ability, etc.?
Specific to your question:
Assuming, as stated in your question, you are using a movie poster and you are copying it from a website.
Is the source governed by legal restrictions? This would be any acceptable use policy or any other communication. Assuming a website for a moment, this would be the case. If so, then these restrictions do apply and you would live within them.
Am I copying the work from an original source? It is possible that you are copying the work from a location which does not have legal rights to the work. In this case, you should seek out legal representations of the original work.
Is the copied portion of work transformative? In this case, substantiality does apply. For example, you would not be able to copy the image, movie poster, and simply resize it to liking. The resizing would have to be substantial such as a thumbnail image. See: https://en.wikipedia.org/wiki/Kelly_v._Arriba_Soft_Corporation As well, for example, if you were to overlay a user or critics rating over the image, that would be transformative. It is recommended that both concepts be applied to mount an appropriate defense.
Is appropriate credit given to the original work? If so, then fair use is emboldened.
While this answer cannot cover all aspects of the fair use doctrine of the U.S. Copyright laws and I am not a lawyer, it does cover quite a lot. It is your responsibility to ensure that copyright law is followed.
