Only way that can be done is for the image to have been larger; Compression ratio could be altered before upload, up to about 30%, that will help to reduce file size, whilst retaining picture size; the whole watermarking process is automatically handled by site software.
I am looking in to variable watermarks. The watermark will be proportionate to the size of the picture. To be honest this a way off yet as I have some more important tasks to catch up on with the site... Mostly due to my absence.