A paused video frame can be a useful thumbnail source, but the most dramatic instant may also contain motion blur or an awkward expression. Evaluate the frame as a still image.
Look around the moment
Inspect nearby frames before and after the action. A hand may be clearer just before movement, while the result may be clearer immediately afterward. Choose the frame that communicates the task without requiring viewers to infer a blurred detail.
Export from the source
Use your editing application's frame-export function when available rather than photographing the screen. Keep the highest appropriate source quality and avoid unnecessary repeated conversions.
Review as a thumbnail
Crop and reduce the frame to its intended display size. Check focus on the main subject, not only the background. If no frame is sharp enough, use a separate photograph of the same genuine setup. Label the source in your project notes so future editors know whether the image came from footage or a dedicated photo.