You can use a still of a purchased video as your album cover, no worries at all.
However (and this is a long shot) if you see another Rotor user online who has used the exact same still for their album art cover... neither of you will be able to initiate a copyright claim (as neither of you own the still) - both of you have non-exclusive rights to that still.
https://rotorvideos.com/rights
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