photoshop workshop - layering and layers. opacity, overlay and screen

there are many forms of layering and layers
one way is to overlay an image over another.
to do this drag a image over another and re size the image to cover the whole of the other image.

now select overlay
 now you can allow the other image to come through, you do this by changing the opacity.

above is how the image may look when using overlay and opacity of 31%


another layering is screen

to use screen change normal or overlay to screen. as seen below.