Start by reading Udemy's materials on the subject at https://support.udemy.com/hc/en-us/articles/229232347-Course-Image-Quality-Standards - it covers most of what I was about to type up for you 🙂
What I usually do is get a simple stock illustration (from istockphoto.com, so I have a real license to use it) that conveys the topic I'm teaching somehow. Then I overlay a headshot of myself trying to look warm and inviting over it, with a bright, fuzzy border around my head to make it pop. Humans are drawn to images of faces, so that's one way to catch their attention when they are scrolling through a list of courses.
You might also look at the course images the courses competing with you are using. Can yours be better? Can it stand out through the use of an eye-catching color that your competition isn't using?
If you need help putting it together, there's always upwork.com. Just be super specific in what you want. But really you only need basic skills in an image editor of some sort.
