The PNGwriter library, which requires libpng, allows you to plot to a 48-bit PNG file, saving it directly to disk. Plotting is as easy as specifying the red, green, and blue values and the x, y coordinates of the pixel. It includes functions for plotting simple geometric shapes (circle, rect, line), plotting text (UTF-8 support for Asian languages), reading the colour of a pixel, reading in a whole PNG file (great for image analysis), plotting and reading in HSV colourspace, and many others that might come in handy.
The Open Source toolkit for Secure Sockets Layer and Transport Layer Security