Hello

I need to change the background image only for tablet and mobile. 
How can I do it?
Here is the page, which is fine in desktop but it isn't in tablet and mobile.
https://gkgo.pt/ginkgodesign/contactos/

In the desktop version I have the hand at the left side of the contacts.
And at the mobile version I need the hand at the bottom, after the contact form. 
It is possible?

Thanks in advance.
Best regards, 

Diana Soares