3.3 Image formats and transparency
Although your scanned image will be rectangular in shape, nearly all mapped geographic regions will have irregularly shaped boundaries. Thus it is preferable to make the background parts of your image transparent rather than a solid background colour which will obscure neighbouring regions. The variety of image formats that are usable with MapServer and their various advantages and disadvantages is a complex subject which we cannot be authoritative about.
We have found 32-bit TIFF (RGB plus alpha layer) or 8-bit palette PNG with a transparent background colour work; you may wish to experiment.
See MapServer raster data (http://www.mapserver.org/input/raster.html) for more information.
Section last modified: 08 September 2010