You can see how best-selling gigs have designed their gig cover image. Take some inspiration and design your gig image then. Image SEO of gig cover is also needed. Basic graphic design principles needs to be followed too. Best way to judge your gig image is, think you are a buyer who wants to hire someone; imagine what all things they want to know or see to click on your gig image. Not just gig image even your gig title should be click-provoking. All the best.