Here, for example, are three photos:
Here's a lovely shot of a beach in Hawaii
Here are Dwayne and Brenda in Hawaii (ok, so they're a bit overdressed..)
You can choose whatever brush size you want to make easy work of the gross erasure. Which size you choose depends on how big your image is that you're working with. This is the easy part; you can do this in less than a minute. You'll end up with:
To see how you've done, choose Layers/view current only and you'll see your foreground image with most of the background erased:
Now use Layers/View All to put it back like this:
Be sure not to use too small a brush. The built-in anti-aliasing feature of the Eraser brush lets you get the results you want without getting too close to the edges of your foreground image.
TIP: If you find you accidentally were too agressive in this step and erased some of your foreground image, don't worry. All you have to do is hold down the right mouse button and use the Eraser brush to put back the original image!
Well, that's it. After a while (and by judicious choice of brush size for tight areas, you'll get something like this:
Oh, and one more thing. Suppose when you're done you don't like the background with the subject foreground. Do you have to do all the work again? Nope. Here's a different background:
We'll bring up the .psp version of the combined image from before:
..and choose Layer/view/Current only to see just the foreground image:
Use [Ctrl-C] to copy the foreground image, switch to the background and [Ctrl-E] to paste it in. And there, with virtually zero additional effort, are Dwayne and Brenda in the forest: