The shelf has to make sense as a set
Shorts can show a custom thumbnail on the channel and in some surfaces, depending on eligibility. When you have that control, use it. A random frame from the middle of the clip is how a series looks like five unrelated videos.
Keep text huge. Brand the series with a repeated color or corner mark, not a watermark across the face. The thumbnail should match the first second so the click does not feel like a bait.