Fiverr Gig Image Thumbnail Size should be 550 pixels X 370 pixels. It’s a minimum size but can use Thumbnail image 1500 pixels X 1009 pixels.For Fiverr Gig image you should be covert into Webp Image to JPG or PNG formate after that you can upload it to thumbnail .
570 x 400 or 1710 x 1200 for good regulation gig image.
we use 1710 pixels width x 1200 pixels height . Image Size: Image size can be up-to 5 MB.
For Converting you can use this website : https://ezgif.com/webp-to-jpg