With Preview alone, you can do the most basic image editing, such as Extract Shape.

Pick Your Image

If your image is in the form image type file, i.e. JPEG, GIF, PNG, etc, you can highlight the file and then choose Open With ▸ Preview (if Preview hasn't been set as your default application) or simply use A.

However, if you want to pick your image from existing Application's icon, you can do these steps :

1. Highlight the icon
2. Copying it into clipboard (C)
3. Open Preview
4. Select File ▸ New From Clipboard (N)

I picked : MacBook Air which I just captured from Apple.

Pick the Object

Let's say you only want the MacBook Air's pic. I'll remove the background (and the envelope) from MacBook Air.

The initial image looks like this inside Preview.

Select Extract Shape Tool

Extract shape tool can be selected from your Preview tools collection. It's under the tool named Select, click and hold on it then a pop up menu will appear. Then you can now select Extract Shape Tool.

Draw the Region

Next, draw an enclosing region using your cursor. After drawing the region, you can adjust it by dragging and dropping some of its nodes.

Press if you've done. The area outside the region will be deleted while the area inside will remain as it is.

Clear Background

If you've noticed, after pressing your working tool is automatically changed to Instant Alpha. This tool is used to adjust the alpha transparency of particular portion of your image.

Click and hold on area outside the MacBook Air and then drag your mouse around. You can see while the percentage is decreasing, the enclosed red region is shrinking. After you released your mouse, the enclosed area will be cut off.

Erase Unnecessary Details

Repeat the step before. But this time, it's for the small details around the main object. It will be a little bit tough and tedious because you need to be careful not to wrongly erase the necessary parts of the image. If you happen to wrongly erase some parts, Z or Undo comes for rescue.

Utilize Zoom Tool

You can use + to zoom-in your image or - to zoom-out followed by clearing out (setting the opacity to 0) certain parts of the image using Instant Alpha Tool.

After 5 Minutes of Cutting Here and There..

You can save it, publish it, or maybe teach other fellow Mac users how to make it.