Easy CMS: spurious frames appearing on image placeholders

I’m using Easy CMS and displaying the placeholder upload symbol on the admin pages without any placeholder text, there being a common note at the head of the page advising the user on how to upload.

On Safari and Chrome this results in just the cloud symbol being shown but on Firefox and Edge there is a frame shown over the symbol, and frames around the already uploaded images. While the frames on the images are no problem I would rather not have these showing on the vacant placeholders.

Firefox / Edge

Safari / Chrome

Has anyone any idea why this should be happening?