I hope this article will help you.
https://www.fiverr.com/support/articles/4410883326481-Gig-image-guidelines-Making-the-most-of-your-Gig-image
Additionally, try to observe some gig images that pop out from others when you searched your category in Fiverr.
So, you can get a rough idea about what type of image you need to create. (Specially Top Rated Sellers)