Doom originally supported saving screenshots in PCX format - a now obsolete format which only supports very basic run length encoding and is not supported by the majority of modern software.
Chocolate Doom retains PCX support but also adds support for saving screenshots in PNG format. This is a popular lossless format that is supported by most modern software and offers good quality compression. Saving in PNG format also performs aspect ratio correction, scaling the image to 1600x1200 resolution.
Another convenient feature is the ability to bind a dedicated screenshot key that is always available in game; in vanilla Doom it is only possible to take screenshots when developer mode is enabled.