Will I lose all existing photos if I turn on photo-syncing to iTunes for the first time?
@IconDaemon is right, best bet is to use Image Capture to download the photos to your Mac:


Connect the phone via usb.
Spotlight "Image capture" or Go To... /Application/Image Capture
Under devices in the image capture window you should no see the
iPhone, select it and it will start loading all the photos.
At the bottom of the window there is a "Import to" selection menu. Choose an iPhotos library, or just select a folder to download to, then click import all


You can optionally choose to delete the photos from the iPhone once imported.

Now that the photos are on your Mac, you can turn on iTunes syncing for photos and select the iPhoto library or folder that you choose to download to.