The PNG (ping) graphics format was developed as a patent-free alternative to the GIF format. PNG or Portable Networks Graphics are raster images that can support millions of colors, like JPG. PNG has variable transparency through alpha channels and has fully lossless compression (that is, saving and re-saving doesn't degrade the quality). These and other features make PNG a better choice than GIF and sometimes better than JPG or TIFF -- it can depend on the specific image and how it will be used.
In the desktop publishing forum, phoenix mentions that in addition to EPS and JPG she supplies clients with logos in PNG format now days in place of WMF, specifically for the transparency. One caveat I would add is that WMF (and other metafiles) can contain both vector and bitmap components. WMF is often used for vector drawings or illustrations intended to be resized. Graphics-clueless clients may expect that all WMFs work the same. That is, a vector image can be resized up and down with little or no distortion. If using PNG images in Word/Office documents, the image may not resize so easily as its vector WMF counterparts making it appear to be a lower quality image than it really is. Additionally, some users may have problems printing PNG images with transparency.
Learn more about PNG:
- "PNG for transparency. WMF for resizing!" Thistle Girl Designs explains formats for their printable collections of images.
- Windows Metafile FAQ In addition to basic definitions and examples, it includes some of the limitations of WMF and EMF formats and tips on converting them to PNG and other raster formats.
- A Basic Introduction to PNG Features The PNG group lays out all the facts about this format.
- Optimizing PNG Images for Web Sites Easy to scan overview of the PNG format with comparisons to GIF and JPG, quirks, and software tools for use with PNG graphics.
- "PNG... isn't that popular in prepress because it doesn't support CMYK." PNG has many great attributes but it's not the right format for professional print work -- or printing in general.
Are you using PNG images? When and where?