You can add as many photos to your gig as you want. You should have the first picture be a clear representation of the end-benefit of your gig. For the other pictures, it depends on the nature of your gig. If you’re a designer, you should show some of your portfolio.
But even if your service isn’t anything visual, you can still get creative and make use of the photo section. For example, if you offer a web-developer service, you could take some pictures of code that you’ve written. Again, look at the top selling gigs in your category for inspiration.
For certain categories, you can also add a cover photo. This one has to be 1100 x 260 and high resolution. You should also add a clear photo showing your face to your profile. This adds a layer of trust to your profile.