Blank picture, 3 x 2 pixels; good for a placeholder when you need it to assume a 3:2 aspect ratio. See Image:Blank300.png for a placeholder which won't take up any visible vertical space, and Image:Blank300vert.png for a horizontal one.
Robert Wahbe, Steven Lucco, Thomas E. Anderson and Susan L. Graham ``Efficient Software-Based Fault Isolation,'' Proceedings of the Fourteenth ACM Symposium on Operating System Principles (SOSP), December 1993, pp.~203-216.