PNG, Portable Network Graphics, is a standard for lossless bitmaps supported by all modern browsers. Among its more notable features is support for alpha channels, i.e. partial transparency, which allows images to be anti-aliased (have smooth edges) and display properly regardless of the background color.
The only real problem with PNG images is that Internet Explorer prior to version 7 does not scale transparency to the web page background, but rather scaled towards the "background color" element that some image editing software can store in the image file. Most image editors pick a default color, often bright.
|
|
| dark gray background (Default skin) |
white background (Monobook skin) |
Notice how the outlines of both icons are nicely shaded (anti-aliased) regardless of the background color.
If you are viewing this with Internet Explorer prior to version 7, you will see a dark gray box around the icon with the white background. This is because prior to version 7, Internet Explorer does not fully support PNG transparency; it will shade toward the "background color" element of the image. This icon has #202020 (dark gray) set as its background color to work well with the default skin, which all unregistered visitors use.