What is the difference in Photoshop CS3 when you are finished doing layers and you want to flatten the image?
The choices are:
Merge Layers
Merge Visible
Flatten Image

I seem to get the same results no matter which one I use? Is there a rule for selecting one over the other?

HI Shelia. These options work the same as in Elements. Yes you get the same results but the first two can be used to merge individual layers. Merge layers, you can select multiple layers that you want to combine to one layer using Ctrl click on each layer to select. Merge visible is basically the same but you don't have to use Ctrl to select the layers. Lets say if you have layers that you have turned the eye turned off and you select the merge visible, these layers will not merge. Flatten image obviously takes every layer and flattens them to one layer whether the eye is on or not and layers do not need to be selected to do so.