Most of the seller's who have many reviews & good portfolio on their gigs, do not add any portfolio image while delivering the order. I'm a Logo Designr, and I don't add any portfolio image of my work while delivering the orders. I've noticed that my first gig are not getting good impression & clicks. So, I again started adding portfolios while delivering every order I've finished recently. And the result was good, my first gig performance is a lot better now than previous condition. So, I believe that this is a ranking factor of Fiverr gigs. You guys should try it, don't leave the portfolio option empty when you'll deliver an order