If an image under a CC-SA license gets used in video - must video be CC-SA?
Yes, that's the core condition of the CC-ShareAlike license. Any derivative or work that contains the image has to be distributed under the same license for the entire video. Even if you create a long video with only one image that is licensed under CC-SA, you will have to use the same license. Using a different license would be a violation to the terms of use. This makes it complicated to use videos and images that were distributed under diverse licences, which is the main disadvantage of the CC-SA license.