I am going to show you how to replace your eyes with cat eyes. A relatively simple task, but in working through the details of this simple task, I will cover a handful of indispensable techniques that should be in every photoshop user's tool kit.
So...let's get started!
Step 1: Gather your images
First things first. Get a picture of yourself. I used my webcam to grab a quick one.
Next find a good high resolution image of a cat's face. Make sure that the cats head is facing the same direction as yours, so that your eyes will line up properly.
My pictures are both full frontal views.
I grabbed my cat picture from sxc.hu. There are other royalty free image banks out there, like the wikimedia commons for example. But, if you are just doing this for your own personal enjoyment, I don't think there is anything wrong with pulling any old image from any old image search. Or, if you have a cat, even better!
Step 2: Copy paste
So.... open photoshop. Select File->Open from the top menu bar and open your image and your cat image. You can select multiple images by holding Ctrl or command and clicking.
Find your cat picture go to the top menu option Select-> All.
Then, in the top menu again Edit->Copy.
You can now close your cat picture by clicking File->Close.
Now your image should be visible. From the menu, Edit->Paste.
If your images are like mine, the cat picture will be waaaay too big.
Next we change the size....