In class this week we made our first (somewhat painful, at least it was for me…) attempts at mixing elements from two different images into one coherent composition. One of the hardest parts about executing this technique effectively is blending the two images without creating something that looks like a second grader’s scrapbook project. There are many ways you can edit various elements from each of the photos you want to combine to make them blend better. One way, is by using the drop shadow tool. Drop shadows create artificial, digitally constructed shadows for selected objects in your image composition. This can be helpful if, for example, you have an image of a landscape that includes a bright sun and you want to insert the image of an animal into the landscape. By creating a shadow for that animal that coincides with the position and intensity of the sun you can begin to make it appear as though the animal was actually in the original photo.
To add a drop shadow to an object within your composition, first select the layer containing that object. Next, select Effects from the windows menu to open the Effects Palette. In the Effects Palette click on the Layer Styles icon and select Drop Shadow from the list. Once the Drop Shadow window is opened you can change the distance from your object, intensity, blurriness, and direction of your shadow. This is where you can play around with the tool for a while to figure out which configuration of shadow components creates the best looking effect. Click the OK button to apply your changes and your generated shadow will appear in your composition.
This article I found from Wiki How illustrates in 8 simple steps the process of creating a Drop Shadow in your photoshop projects.