You do not need Photoshop or Gimp anymore and you do not have to work with layers.
Just take a screenshot of your application and input it to Screen maker.

Screen maker supports 5 kinds of resolutions:
320x480
480x800
480x854
720x1280
800x1280
landscape mode

After finished work, you can see your image or share it.
Screen maker - free
Screen maker - paid

In #1 post I have posted presentation how the final image created by Screen maker looks.

1.0.6 released
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changed layout

1.1 released
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- better 720x1280 phone image
- reduced size
- corrected layout

New phone image examples:

1.1.1 released
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- image rotation added
- new icon

Now you are able to rotate the device, 2 examples below:

1.3.9 released
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screen reflection for SGS3

[SIZE="4"]Examples[/SIZE]