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 put this into the Screen maker.
It will do the best screenshot of your application, ready to post on your website or blog.
In short, this application allows to connect 2 images: screenshot and smartphone image, the result is image ready to post on website or blog.
Screen maker supports 18 kinds of resolutions:
480x800, 480x854, 480x600, 480x1024
640x960, 600x800, 600x1024
720x1280, 768x1280, 800x1280
After finished work, you can see your image or share it.