Can I use a still image from my video for my album art cover?

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