Zewnętrze and other 65,000 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
The icon is the shape of a rectangle with the top line missing to complete it. The top of the rectangle is complete by a partial triangle with no bottom side to it. Inside the rectangle is a square at the upper left corner and another rectangle at the bottom right.