You can set default images or set default to 'No' under the certificate site settings. (Found here: Site administration / ► Plugins / ► Activity modules / ► Certificate).

It appears that when you have the default set to 'No', but that images are uploaded for that field, when creating a new certificate, the default is set to the image. Despite being set to 'No' for default within your Site admin certificate settings.

1. Go to Site administration / ► Plugins / ► Activity modules / ► Certificate and see that Default Signature image is set to 'No'.
2. Upload an image for a field, for instance, Signature Image.
3. Make sure the default for Signature Image is still set to 'No'
4. Go to course and create a new certificate.
5. See under Design Options > Signature Image that the uploaded file is set by default. You will need to go in and change to 'No' or to another image.

It seems expected that if the Certificate site admin defaults are set to no for certain images, that when you create new certificates in courses, that the corresponding image fields under Design Options should say 'No' and that you would need to change to an image.