MMS and other 68,800 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).
It is a speech bubble square with rounded corners. Inside of it are 2 triangular mountains (one slightly larger and in the foreground) and a circular sun, somewhat like a stereotypical landscape photo.