Hi Bikerbob
McGilli is right - the reinstalled iTunes on your refreshed hard drive will view the iPad as being synced with another iTunes Library, ask you if you want to sync it with the new Library and promptly delete all your photos off of it.
However, you can still rescue them! You can either invest in a third-party iPad file explorer app for OS X, or, if practical depending on how many photos you have on it, you can email them all directly from the iPad Photos App to your own email account when you've set it back up on your iMac, save them out and add them to your new iPhoto Library.
Hope this helps!