Presumably the question is whether the "sharealike" requirement of CC-BY-SA would "contaminate" the copyright of the dissertation. If you are OK with having your dissertation under CC-BY-SA, and your university and advisor are OK too, it is not really an issue. (Your university and/or advisor may impose restrictions on what goes into your dissertation. Follow them even if unrelated to copyright and even if they seem nonsensical.)
If not, in the US (and most Western jurisdictions), there is some form of "fair use" doctrine under which critical discussion allows to reproduce otherwise-copyrighted material, sometimes with a particular notice, indication of source etc. Under this, whether the image is CC-BY-SA or any other license (or no license at all) is irrelevant - you reproduce it under fair use or the local equivalent. Again, check your university is OK with this.
(On a side note, many scientific journals will refuse that you reproduce graphs from other papers without written permission, even though that use would usually satisfy fair use requirements. That is because being sued is costly even if you are guaranteed to win.)