How do I find out the URL for an image in my assets? In order to direct Paypal to an image of a customised button

I would like to customise a Paypal button. There is an option in Paypal to use an image of a customised button that already exists. To do this Paypal needs the URL of the image...I think this should be something like...

mywebaddress.com/assets/name-of-image  

Please could you advise?

Thank you,

Rebecca