Raster support in ArcGIS is built on an open structure based on DLL technology. With this open structure, users can add their own format support into ArcGIS by creating a customized DLL and plugging it into ArcGIS. The customized raster format, created by implementing some predefined interfaces, will be treated the same as the other built-in raster formats.
At ArcGIS 8.2, the following raster formats are supported "out-of-the-box": ERDAS IMAGINE, GRID, GRID Stack, TIFF.
Also supported are:
ADRG, BMP, CADRG, CIB, DTED, ERDAS LAN, ERDAS GIS, ERDAS RAW, ERMapper, ESRI BIL/BIP/BSQ, GIF, JPEG, MrSid, NITF, PNG, SDE Raster.
Rasters stored in a format other than those listed here require the user to build a customized dll.
Since ArcIMS also adopts the same open structure for raster support, this customized raster DLL can also be plugged into ArcIMS. This makes serving the customized raster format through ArcIMS possible and easier.