for seo of gig you have inserted keywords on title, keywords section, pricing page, just write down your targeted keyword everywhere, you don't need to select keyword in one section, I recently started working on fiverr , I have observed one thing after doing experiments every day.
like In title I used to do:
example:
"I will build react website"
here my gig it will rank your gig in different three keywords like when user search "React Website" it will be ranked there, it can be rank in "react" and "website" too.
for image optimization just name your images with your targeted keyword, for better ranking use keywords in title and copy your title and use that as name of images.
I have tried adding tags on images, but haven't got that much outcome, naming images with title or keywords are sufficient for that.