It is possible to target more specific elements of the page. Can you isolate phone mockups into page sections of their own? When I say isolate, not have any other non-mock up images that you don't want to get rounded corners in the same sections. Text and other elements is fine of course. Isolating the mockups in this way would allow for us to target CSS to those specific sections.
The other more tedious way would be to target specific image blocks.