The iPad can hold both photos you've taken using the device itself and photos you've synced to it. The process works a bit counter-intuitively, however. Syncing photos from a folder to your iPad for the first time is easy enough, but if you do it a second time with a different folder the photos you originally synced will be deleted. The issue is that you can only sync photos from any one folder at a time -- if you don't want previously-synced photos to be deleted, you'll need to keep them in the folder you synced and place any other photos you want to sync in that folder as well.
Copy all photos you want to sync to a single folder, including ones you've previously synced. If you want, you can create sub-folders inside that one folder to keep the photos organized -- each folder will be an album on the iPad.
Connect your iPad to your computer and, if it doesn't open automatically, launch iTunes.
Click the "iPad" button in iTunes and select the "Photos" tab. Ensure the "Sync Photos From" option is enabled and select "Choose Folder" from the drop-down menu. Select the folder you created in Step 1.
Click "Apply" to start the sync process.