An equirectangular image is the image format supported by the RCX platform.
What equirectangular projection means
Equirectangular projection is a technique for mapping the surface of a 3D sphere onto a 2D plane. Below are some examples of equirectangular images.

Features of an equirectangular image
The technical aspects of an equirectangular image include an aspect ratio of 2:1, the width of the image is twice the length of the height of the image.