Using masks in Photoshop to create images
To create the image here I used three subjects then using masks in Photoshop merged them into one layer. First I photographed the hand in front of a white background, the water splash, and lastly I had an image of a goldfish that I isolated from the background with the selection tool, it really wasn't that hard.
Next I assembled them in order, hand as background, water splash, and lastly the fish. Then I just activated the water splash layer and added a "mask" ( it's in your layer pallette and looks like a rectangle with a circle in it) and filled with black, that makes it disappear, use a white soft brush to "paint" the splash back in and place it in the hand where you want, if you make mistake brush with black to make it disappear again. Do this with the rest of the images until they look just right. Place the fish where you want it and use Transform to resize to the size you want and voila! You've just created a new image!
This can be a lot of fun and you can get very creative!