Transferring Pictures to iPad

What's the best way to transfer picutres from the hard drive to the iPad. Not extremely familiar with itunes, but when i upload them into itunes and then download them to the ipad it puts on the new photos but deletes the existing ones on the ipad.

I'm trying to transfer over certain folders that contain several pictures from my hard drive to the ipad. e.g.-baseball folder, vacation folder, school folder, etc. Any advice is greatly appreciated.

Thanks.