I tried on three different computer running Mojave, Big Sur and Monterey. I then thought it could be the latest version of Mailspring but the past two versions and no luck. Only my laptop, that already had Mailspring installed with a signature image, is working properly.
I see the issue on Ubuntu as well, but it will take a PNG no larger than 64px by 64px in size. I suspect there’s a size limit but the servers aren’t responding with the correct response.