How to capture whole screen in Corona SDK

You can use the following lines of code to capture the device screen in Corona SDK. This will save the screenshot as: Picture 1.png, Picture 2.png, etc in the sandbox, and will show it on the screen with half size(if you do not need this, you can remove it by calling captured_image:removeSelf() and captured_image=nil). It can save upto 10000 files.

Syntax:

    local function captureDeviceScreen()
       local captured_image = display.captureScreen( true )
       captured_image:scale(.5,.5)
       local alert = native.showAlert( "Success", "Captured Image is Saved to Library", { "OK" } )
    end
    Runtime:addEventListener("tap",captureDeviceScreen)


And if you want to save the image with desired name, then you can refer the following post:

How to mail a screen captured image using corona SDK