Recover pictures deleted by syncing iPhone from different photo library
iDevices can only by synced to one computer running iTunes.
Syncing to a new computer/iTunes will erase all previous data.

If you still have the other computer that the iPhone was last synced to, then you might be able to retrieve them from your last backup. Alternatively, check your iCloud storage.

It would have been far safer to click Cancel &amp; instead copy the previous photos to the computer using Photos.app or Image Capture, Mac or Photo Gallery [part of MS Essentials], PC.