Hi Dayle
I also wondered the same thing until I took the "Blending Modes" class. That's one way to do it - by using the blending modes in your program.
Somewhere near your layers palette on the right side, (mine is above it) you should see the words "Normal" and "Opacity" - each with a drop down menu.
If you play with those, you can get some really cool effects! For instance, "overlay" would be like making your top layer (photo?) into an overlay.
Make sure your top layer is selected, then click on "overlay" (in the "normal" drop down menu) and then click on "Opacity" and play with the slider to see how you want it to look. There are many choices and combinations to play with.
I've also used Brandy Murray's Background Blenders and Background Blenders - Masks for photos. They're great!
Hope that helps.. This is a short explanation but I"m sure someone with more knowledge will come along soon.
If you'd post what exact program you are using, that would probably be helpful for more specific info.
Good luck and have fun!