script to preview two .png s side by side?
Open the two files with Preview, then run the following to position the windows:

tell application "Preview"
    activate
    set the bounds of the first window to {0, 0, 700, 1000}
    set the bounds of the second window to {700, 0, 1400, 1000}
end tell


Customise the coordinates as you wish depending on monitor resolution.
Syntax: {x1, y2, x1, y2}

Enable AppleScript in Preview in OS X ≤10.8 by quitting Preview and running:

defaults write com.apple.Preview NSAppleScriptEnabled -bool true