There are two kinds:
Asymmetrical... Even though it's not the same on both sides, the larger fish is still balanced by the presence of the smaller ones. When I'm doing something asymmetrically balanced, my rule of thumb is to make the elements different sizes. In this case it's the fish. The large fish needs something there to counteract it and the small ones would feel lost without the big one. Asymmetrical balance is really fun. Fun, and easy, and it almost always looks good. It's almost always exciting to the eye.
On the other hand, there's the second type of balance, the one we all probably think of first when we hear the word balance:
Symmetrical. It's pretty straightforward. Both halves are mirror images of each other. Symmetrical balance can be very beautiful. The only pitfall is that it can go toward the staid, unexciting side of things. My favorite way to make a symmetrical image work is to make it intricate or bold in some way so that it isn't a snooze fest. If your style is traditional, chances are you'll like this type of balance. If your style is more eclectic or modern, you may find it too boring for your tastes. My favorite way to make a symmetrical image work is to make it intricate or bold in some way so that it isn't a snooze fest.
I made these two images tonight just for this post and I've got to say, my favorite is the second one. It'd make a good pillow.
By the way, I'm working on something REALLY fun and mostly FREE! I'm pretty excited, I can't wait to finish and post it here really soon. For tomorrow, though, I need advice on what to do with my entryway. I'm posting photos. It's pretty scary. Check back if you like to be frightened.