Blending modes are useful way to change your image. When adding highlights and shadows, they can be helpful to making things look more realistic. Im going to explain the most important and useful ones: Multiply, screen, overlay, soft light and hard light. All of these can be found on your layers palette.
To illustrate the effects, I am going to use three images, a leaf, a black and white texture of wood, and some abstract colors. The texture you want to add must be on a layer above the picture you want to affect. From there, the rest is easy. Click on your blending mode drop down menu and select away.
MultiplyMultiply makes your white area transparent and your black areas a little darker
ScreenScreen does the exact opposite. Black areas become transparent and white is enhanced.
OverlayOverlay is usually the most useful. It can be especially good in adding textures such as stone to flesh. It can be described somewhat as a if you are laying on your texture at lower opacity, but with more realistic results.
Soft lightThis one is like overlay but less strong
Hard lightBlends the layers together and then adds what seems like a harsh spotlight on the whole thing. It is most evident in the wood texture.
There are other modes but these are the most useful ones. The other ones are easily seen by exploration and playing. Lets take a realistic example or where this can be useful. Lets say I want to add an interesting lighting effect to an image. Heres my original and a layer above it filled with a black and white gradient.
Im going to apply the blending modes in the order they were presented above *except soft light, it didnt have much effect. Nice effects arent they?
You can find all kinds of ways to use blending modes. If you duplicate your original layer, blending modes can change how things look (ie. without using an outside texture). Hard light is especially useful if you want your original image to look brighter lit. You can also alter how blending modes affect the image by desaturating the lower layer, changing the opacity after blending you name it. The possibilities are really endless!