There's the image you want (be it an object or a person) on a plain white/black/other background. These are ridiculously easy to get rid of and here's a tutorial showing you how.
Step 1: Find a base image with a one-colour background - in this case, the charming, blue-eyed Mr Craig. Here, we have a publicity photo on a white background. The essential thing is that it is a single colour (in this case, white has the html value of #ffffff - look it up if you don't understand. It's very important to understand this principle.)
Step 2: Make a copy of the background layer by pressing the "Copy" button (blue arrow). (In case you don't know, this is because you can't make original backgrounds transparent.)
Step 3: Make a white layer. Just press the "new layer" button (blue arrow) and then select the "white layer" option (pink arrow). If your background is black, pick a black layer instead. If your background is #f9CC22, then make a new layer of colour #f9CC22, etc..
Step 4: Move white layer down (blue arrow) and delete the ORIGINAL background layer (pink arrow). Make sure you've selected the right layer for each of these operations. If you delete/move the wrong one, don't panic, just press Ctrl+Z to undo. When you're done, you want your white layer under your copy of the "Daniel" layer.
Step 5: Make the white vanish. Okay, the exciting bit. Click Layer-->Transparency-->Color to Alpha.
Step 6: The Colour-To-Alpha dialogue box appears. (You can enlarge this image right now by clicking on it. Have a look at my screen capture and the clarifications I wrote on it.)
Basically, you want to delete any part of the image which is the same colour as your background. In this case, I just have to pick white (blue arrow), and then press okay (pink arrow).
Step 7: Selecting the outline. Okay. Make the underlying white image invisible for a moment (blue arrow).
"ARGH!" (I hear you cry.) "Daniel is transparent! I don't want that!"
This is the really clever bit.
When you made the white transparent, you made ALL the white in the image transparent, including things like skin or clothes. Here's how you fix it:
Right click on your "Daniel" layer. Now pick the Alpha-to-Selection option (pink arrow). You have just selected everything that is NOT transparent in that layer.
Step 8: Close-up. See the marching ants around Daniel? It means he's the only area selected in his layer.
Step 9: Deleting the inverted selection from the white layer. Without changing your selection:
- Make "Daniel" invisible (blue arrow).
- Make sure you've highlighted the white layer (pink arrow).
- Invert the selection (Ctrl+I, or see previous tutorials).
Step 10: Deleted Selection. You should have something that looks like this. Any bits that are still transparent (sometimes happens) can be painted in quickly.
Step 11: Flatten image. You want to merge your white layer, and your top image. There are several ways of doing this. My favourite is to right click on the "Daniel" layer and pick the "Merge Visible Layers" option (blue arrow), but the two pink arrows are alternatives.
Now that you have your cut-out, do anything that your whimsy dictates. :)