I would like to use an image from Wikipedia, licensed with Creative Commons Attribution-ShareAlike (CC-BY-SA) license, as a part of my video. The image would appear in its original form, possibly scaled. Does this make my video a derivative work which must be "shared alike" or is it enough credit the image properly?
ShareAlike photo being used unmodified in a larger work. Unless the larger work would be considered an adaptation of it, using a ShareAlike photo as a separate element within it does not require original materials in the larger work to be ShareAlike or compatible. The larger work may be licensed under any terms.
This would suggest that simply displaying the image in your film would probably not require you to apply an SA licence to your film. I imagine you would still be expected to cite the source according to standard CC requirements.
However note the disclaimer at the head and foot of the page that this is not legal advice.