Image editor features>>Selecting and merging of images

Many graphics applications are capable of merging one or more individual images into a single file. The orientation and placement of each image can be controlled. The two images shown here were once individual studio portraits.

When selecting a raster image that is not rectangular, it requires separating the edges from the background, also known as silhouetting. This is the digital version of cutting out the image. Clipping paths may be used to add silhouetted images to vector graphics or page layout files that retain vector data. Alpha compositing, allows for soft translucent edges when selecting images. There are a number of ways to silhouette an image with soft edges including selecting the image or its background by sampling similar colors, selecting the edges by raster tracing, or converting a clipping path to a raster selection. Once the image is selected, it may be copied and pasted into another section of the same file, or into a separate file. The selection may also be saved in what is known as an alpha channel.

source : http://en.wikipedia.org/wiki/Digital_image_editing#Basics_of_image_editing

A popular way to create a composite image like this one is to use transparent layers. In this case, the "background image" shown at left was placed as the bottom layer. The layer marked "Original Image" at left was then added as a second layer in a multi-layer document. Using an image layer mask, all but the girl are hidden from the layer, giving the impression that she has been added to the background layer. Performing a merge in this manner preserves all of the pixel data on both layers to more easily enable future changes (such as adding the second individual) in the new merged image.


Orginal Image Background Image


Merged Image