If the image is in the public domain, or if it is licensed under terms that allow distribution without attribution, then including it in a post is fine. There aren't many images out there in the public domain; the main sources are old images whose copyright has expired (rare in an SF context) and images made by the United States government or federal agencies (e.g. by NASA). Providing an attribution is still often regarded as the right thing to do, but it is not legally mandatory.
If the image is under a license that allows distribution, but only with attribution, then adding a missing attribution can make an image legal, irregardless of whether fair use applies. These are the terms of the CC BY-SA license that all content on this site is under, as is Wikipedia's text — note that not all images on Wikipedia follow this license.
The third way in which an image can be legal to include in a post is fair use. Invoking fair use is trickier, because the exact limits are defined by jurisprudence. The factors that determine whether a use is fair are:
- the purpose of the use — a Stack Exchange post is borderline, since the poster does not derive any direct benefit, but Stack Exchange itself is not a non-profit enterprise.
- the nature of the work — this provision lowers the bar when distributing works that are essentially documentary or designed to be reproduced (e.g. a recording of a historical event, a politician's speech).
- the amount of copied material — short extracts or reduced-quality reproductions are more likely to be considered fair use than the original work in all its glory.
- the effect on the work's value: parody and criticism are legitimate, as are in many circumstances copies made for the private use of the copier (such as recordings of a television broadcast intended for delayed viewing), competition is not. A review (whether positive or negative) may legitimately cite small portions of the work that it is reviewing; these portions should be relevant to the points made in the review.
Adding an attribution does not in itself make a use fair, but it can be a contributory factor. The attribution mainly impinges on the last point: if you do not attribute a quotation, then it looks like you are appropriating, and thus you may be competing with the original work that this quotation is from. Adding an attribution makes it more apparent that you are reviewing the work (and perhaps giving an incentive to the reader to consume the original work).
P.S. Moderators are not copyright lawyers. It is not possible for us to have an authoritative opinion as to whether a use is fair. It is easier for us to rule on plagiarism — any material included in a post that is apparently not the work of the author of the post may be considered plagiarism and deleted. Therefore, it is recommended that you always correctly attribute material that you did not originate. In any case, every poster retains all legal responsibility for their content. If a work that you own the copyright to is misused on Stack Exchange, the proper procedure is to file a DMCA takedown notice to the contact address mentioned at the bottom of every page.