Most course thumbnails fail for the same two reasons: they try to show too much, and they look fine at full size but disappear at the size they actually appear on screen. In a course catalog or social share, your thumbnail is competing at roughly 300 pixels wide. Design for that context, not for a billboard.

High contrast between the background and text is not optional — it is the whole job. A dark background with bright text, or a light background with near-black text, will always outperform a mid-tone palette that feels 'subtle.' Subtle does not get clicked. Legible does.

Use one dominant visual element: your face, a screenshot of the outcome, a bold number, or a clear icon. Faces tend to convert well because they signal that a real person is teaching and that there is accountability behind the content. A screenshot of a result — a finished design, a chart, a code output — works well when the transformation is visual.

Keep text to a maximum of five or six words. If you need more to explain the course, your title probably needs reworking rather than your thumbnail. The thumbnail's only job is to make someone curious enough to read the title. Once they read the title and description, they have everything they need to decide.

Test by shrinking your thumbnail to 100 pixels wide and squinting at it. If you can still read the text and recognize the main image, it will hold up in the catalog. If it turns into a blur of competing elements, simplify until it does not.