The editor uses Equirectangular Panoramic Images for the background elements.
The supported file formats for the Equirectangular Images are: jpg & png Equirectangular Images have an aspect-ratio of 2:1 and the recommended resolution for the same is 4096 x 2048.
The file size of the Equirectangular Image also affects the load time of a scene in the experience you create. Hence, the recommended file size for an Equirectangular Image for faster loading time is 1MB.
You may use online compression sites like tinypng to compress png and squoosh to compress jpg images, for a lossless compression.