One of my favorite techniques for removing (i.e. make transparent) dark or light areas of an image is to use the blending sliders available inside the Layer Style dialog box within Photoshop. These blending sliders are easily overlooked in pursuit of the more commonly-used Layer Styles available in this dialog box.
To demonstrate this technique, I am going to combine the water image on the left with the shell image on the right. I want the end result showing the splash and runoff of the water over the shells – giving it more of a ‘being on the beach’ feel and almost hearing the waves going in and out.