Portable Network Graphics (.png). A bitmapped image format that employs lossless data compression defined in RFC 2083. It was created to improve upon and replace the GIF format as an image-file format not requiring a patent license. PNG supports palette-based 24-bit RGM colors and grayscale images. It does not support color spaces such as CMYK.

For more details see the Wikipedia article Portable Network Graphics

For the digital library, convert PNG images to PDF for the image version and upload the original PNG as the archive version.