Crosspixel

1. Click extension's icon
2. If design.png exists, it will be placed over the page.
3. If there is no design.png file, just drag and drop image onto the page – image will be placed over.
4. To change opacity use left click and right click.
5. To remove image placed over page, click extension's icon.

Image is centered horizontally on a blue translucent background. The width of the superimposed image in pixels displayed in the Developer Tools console. Change the width of the window is conveniently with Developer Tools enabled - in this case in the upper right corner shows the size of the window in pixels.