Do you mean Jan Kuiper?
I see. It would be best for me to see the piece, the signature and the back ( to see frame and canvas construction, helps determine age). You can add pictures by using the ADD FILES feature that should be on your screen.
There should be the words ADD FILES on your screen in red somewhere (it is in different places if you are on mobile or desktop).
If that does not work, you can email them to***@******.*** and put 'for rarewares in antiques' in the subject line so they will get forwarded to me. This way takes longer, so thank you in advance for some patience.