Changing Background

Original Image:


Technique 1: The Quick Mask Technique

If you've never used Quick Mask mode in Photoshop, you're missing out on a huge time-saver! Quick Mask mode lets you use brush, pencil, paint bucket, etc. tools to paint an "inverse selection" using shades of grey and black. (Black identifies parts that won't be selected; grey identifies parts that, when selected, will have some level of transparency.) When you switch back to Normal mode, the "clear" parts of your Quick Mask are selected so that you can apply changes. We'll use Quick Mask mode to paint the background of the image, then apply a Layer Mask to hide the background.
Click the Quick Mask icon in the tool palette to switch to Quick Mask mode.

Using a fairly large brush, with the foreground color set to black, I'll start to block out the background. You'll notice that the masked areas in which you paint will turn pink.

The large brush size allows me to color in the main areas of the background fairly quickly.

I can then use smaller brushes to fill in the details around the edges.

Eventually, I have the entire background colored in pink:

Tip: You may change the foreground color to white if you want to "paint back" parts of the object.
Switch back to Normal Mode, and you'll see the selection. Because everything that's painted in is not selected, the final result is that the bee is selected.

At this point, I click the Layer Mask icon at the bottom of the Layers Palette to create a layer mask, which essentially isolates the bee from the background.

So, here's the final picture:

And in summary:
Quick Mask Technique:

1. Switch to Quick Mask mode and paint everything you want to get rid of.

 

1. Switch back to normal mode and use the resultant selection to make a Layer Mask.

Total time: 6 minutes
Notes: A great method, because it gives you precision-control over what gets selected. This method works well for any picture -- solid background color or not. The only unfortunate aspect of this method is that, for some objects, you may see a "color halo" where the edges have picked up on some of the background color. (One way to get around this is to take a little more time and paint around the edges with a grey airbrush to make the edges a little transparent.)

Technique 2: Pen Tool Technique
Use the Pen Tool to create a path that outlines the object, then use the path to create a selection for a Layer Mask.
Choose the Pen Tool. In the horizontal options bar, make sure that "Paths," not "Shape layer," is selected. Then, start using the Pen Tool to create a path around the object. I find that it's helpful to zoom in for greater detail.

Here's my completely outlined bee. You can see all the different points that I created using the Pen Tool, as well as the resultant path.

Open the Paths palette and Ctrl-click (Command-click for Mac) on the path layer. This creates a selection from the path. In the diagram, you can see the dotted line that shows the selection.

If you have a pretty solid-shaped object, you can skip the next step. Because I have a fuzzy, plush object, I'm going to feather my selection slightly so that the edges will be a little blurred. I choose Select > Feather and make a Feather Radius of 1 pixel.

Finally, I click the Layer Mask icon in the Layers palette to create a layer mask from my selection. This hides the background and isolates the bee.

Here's the final picture:

In summary:
Pen Tool Technique:

1. Use the Pen Tool (set to "Paths," not "Shape Layer") to create an outline of the object.

 

1. Ctrl-click (Command-click for Mac) the path layer to make a selection.

 

1. Use the selection to create a layer mask.

Total time: 3 minutes
Notes: This method works for any object, regardless of the background, and is also quite precise. It's reasonably fast if you're comfortable with using the Pen Tool; otherwise, this method might take a long time! This method is probably better for objects that have crisp, hard edges, and, similar to Quick Mask mode, you may have a slight color halo if the object has picked up some of the background colors.

source: some excerpts were taken out from an article by  Corrie Haffly, which can be found here: 5 Great Background Masking Techniques in Photoshop