Thank you all for your suggestions. They have been really helpful. I agree regarding the importance of clearly showing the actual image/print size, unfortunately I will have to do that for each image since I crop a lot and do not use fixed ratios, so it will a bit time consuming. And yes, I must also show metric size, that is what we use here in Italy
So far I used to place my signature close to the paper border, I see now that it would be better to place it close to the image border, thus allowing the matt window to show it.
Thanks again guys!