I am a new seller. I would like to know can I add testimonials of my previous clients
& can I add certificates from udemy on my gig video
The reviews posted by your buyers are the testimonials for your service. There is no place to add online course “certificates” themselves, however, you can list that you completed certain online courses on your Fiverr profile.
would it be appropriate to have one as one of the gig images?
Yes, I am talking about the reviews…can I use them on gig videos
That I cannot viably answer. If the TOS doesn’t say anything about this, then it might be best to contact Customer Support directly, and ask. I, myself, don’t know who retains legal right to the reviews. I don’t know whether this would be the buyer, Fiverr, or you, as the seller who earned said reviews. Contact CS, and they can provide the best answer.
I would not recommend it. Fiverr intends the gig images to be for use in representing your work/portfolio files. Buyers expect to see samples of your work in those images/gallery slots. Uploading a picture of an online course “certificate” may be off-putting to buyers. Given the purpose of the gallery, it is possible that it may not appeal to Fiverr’s review staff as well.
It’s also worth noting, an online course “certificate” is not proof of any skill or experience, as the gallery is designed to show. The gallery should show examples of your previous work, since that is what buyers expect to see. If you don’t have previous work to show, create something new (for fun) with the skills you presumably gained during the course, upload that creation, and use that as your work example.