Cory Bohon on using vector images in Xcode 6:
“…Xcode’s support for creating your PNG files at build time from a vectorized PDF. I share with you exactly how this is done.”
The trick is including vector-based images in a PDF file and letting Xcode create @1x, @2x, and @3x PNG files during build time. This should be of great help as additional devices/resolutions are released.