Q: how do i keep photos already synced and transfer additional photos to my ipad?

Syncing via iTunes means that you aren't syncing photos as much as a folder's content. To add more photos, put those photos into the folder you selected on your computer then sync.

Another way, depending on how many you have, could be to simply mail your photos to yourself and save them from mail.

To keep previously synced folders of photos keep them selected in iTunes when preparing another sync. All selected folders will sync to the iPad. Any deselected or unselected folders will not sync to the iPad and if they were previously on the iPad they will be removed during the sync process.

So the simple answer is to make sure all folders you want on the iPad are selected in iTunes before syncing.