Picture size

Hello,
I’m running a charity website with a default picture size of 1024×768 and doing fine since many years, but by now I would like to upgrade to Twenty Fourteen and be more responsive.

Question: Would I need to resize all the picture sizes and upload them again to fit the 1038px in witdh, so I’m able to use them as a featured post randomly without getting the few picture gaps?

Many thanks in advance for any hint.

: – ) Dirk