Some sites like Instagram were/are specifically restricted to square images only (I think that’s changed now but I don’t use instagram)
When images are very wide or very tall, apps will often fill to a differently aspected thumbnail, cutting off important parts of the comic like dialogue. My Lemmy client (Connect) does this, for example. If your base image is square, then the thumbnail will also always show the whole image.
A square image is a compromise which is fine on both mobile and desktop and is a middle ground everyone can tolerate.
I can think of a few possibilities
Some sites like Instagram were/are specifically restricted to square images only (I think that’s changed now but I don’t use instagram)
When images are very wide or very tall, apps will often fill to a differently aspected thumbnail, cutting off important parts of the comic like dialogue. My Lemmy client (Connect) does this, for example. If your base image is square, then the thumbnail will also always show the whole image.
A square image is a compromise which is fine on both mobile and desktop and is a middle ground everyone can tolerate.