A really strong thumbnail strategy is building branded thumbnails

Kurzgesagt is a really good example:

Every video has the same dark background, the same illustration style and the same warm colour style. Once you've seen a single Kurzgesagt thumbnail, you can recognize them instantly.

Branded thumbnails work because:

• Familiarity builds trust

• Trust drives clicks
  Our brains are naturally drawn to what we are familiar with, specifically to who we know, like, and/or trust. (Social Hacking)

Implementing branding in your thumbnails doesn’t have to be complicated:

• Use your face

• Use the same font style

• Use the same 2–3 core colours every time

• Keep a consistent background style

• Place your logo in the same corner
  You won’t always be able to do all of these (especially depending on your niche) but using one of these tactics consistently will make a noticeable difference

It’s about being recognizable