I shot some random seagulls in a parking lot. The background was awful, but the birds stood still. A year later, I shot a Canadian Navy ship in Halifax, Nova Scotia. Nice picture, but needs something. Maybe that seagull. So, let's put him there. (The gull and the ship are available for download at the usual source, if you would like to work along. Look for gull.jpg and ship.jpg.) Figure 20.2 shows the gull.
Select and copy the gull, then open the ship picture and paste the bird in. Figure 20.3 shows the gull after this step.
After reducing him down to a more realistic size, using Edit
There's no way to determine which settings to use, other than trying them to see what works best. The only one I could set arbitrarily was the angle. That's because I knew where the light was, and shadows are usually directly opposite the light source. Setting the other parameters gives you a chance to play with the shadow, to make it harder or softer, bigger or smaller.
Finally, a tiny bit of dodging on the gull's back and burning on his face to bring out the detail will help a lot. Figure 20.5 shows the finished photo. Notice how adding the gull helps you see how big the ship really is.
It's extremely simple to paste one opaque image over another oneyou just do it. Transparent images are harder to work with, although Photoshop makes it a little easier by giving you opacity controls.