Yes, most camera batteries are rated in number of pix taken, not hours. Your manual will tell you how many pix your camera will take, on average, for a set of batteries.
Yes, 2500 mAh is a spec that batteries have. It says that the battery will provide 2500 milliAmperes for an hour into a standard load. You probably only care that you can buy such a battery.
Your manual tells you on page 087 that you can use Ni-Mh rechargeable batteries. These can be purchased lots of places, I got mine at WallyWorld, don't forget to get a charger at the same time, buy a kit that comes with a charger and 4 batteries. Two to be charging while you use the other two. Most brand name Ni-Mh rechargeable batteries will be rated at 2500 mAh, and at least these Energizer ones are.
http://ec1.images-amazon.com/media/i3d/01/A/man-migrate/MANUAL000068982.pdf
Download your manual from here
http://downloadcenter.samsung.com/content/UM/201007/20100727142406500/ES28_English.pdf