USING SCREEN GRABS FROM TRAILERS

Hi, I'm making a book video trailer using stock and the author has asked me if it's possible to use screen grabs from the trailer to use for things like stills ads on their author website/social media/marketing campaigns etc.

e.g. A screen grab still with something like, will he catch the thief?

Can this be done? How would it work if it can be done?

Greatly appreciate any wisdom/knowledge.

Thanks