2D Bitmap Specifications:

BIFF - Notes on the XITE 3D file format
BMP - MS-Windows bitmap format
BW - SGI Black & White Image File Format
CGM - Computer Graphics Metafile
CGM Overview & Description
CGM Technical Reference (WordPerfect 5.1)
CGM Information, Examples, and Source Code [Link]
DRAW - Acorn's object-based vector image file format [Link]
DWG - AutoCAD drawings file format information [Link]
FAX - The Group 3 Facsimile standard
CCITT Draft Recommendation T.4
CCITT Group 4 pointer
DCX Format (graphics format for fax)
EPSF - Encapsulated Postscript Files
FIG - The FIG V3.1 file format (used by the xfig utility)
FITS - Flexible Image Transport System
GIF - Graphics Interchange Format
GIF87a Specification
GIF89a Specification
GIF Compression
GL - Another animation format
HDF - Hierarchical Data Format [Link]
ICC - Used for Kodak printer
IFF - Interchange Format Files(e.g. ILBM & PICS)
JPEG - JPEG File Interchange Format (V1.02)
MIFF - Machine Independent Format
NAP - The NAPLPS objected-oriented format [Archive]
netCDF - The network Common Data Form [Link]
PIX - Used by SGI Alias|Wavefront products
PCX - Used by PC Paintbrush
PCX Specification
Additional PCX Information
PNG - Portable Network Graphics Specification [Link]
PBM+ - Enhanced Portable Bitmap toolkit
PBMPlus overview man page
PBM (monochrome bitmaps: 1 bit per pixel)
PGM (greyscale images)
PPM (full-colour images)
PNM (general image format)
RLE - Utah Run Length Encoded Format
RAS - Sun Raster File Format
RGB/RGBA - SGI Colour Image File Format
SLD/SLB - AutoDesk Slide File Format
SLD Slide File Format Specification
SLB Slide Library File Format Specification
SPRITE - Acorn's bitmap format for their RISC OS [Link]
TGA - Targa File Format (this link incomplete with mistakes at Image Descriptor)
Targe Spec Table of Contents (PostScript)
Targa Specification (PostScript)
TIFF - Tag Image File Format
Tiff Revision 4 Info
Tiff Revision 5 Info
Tiff Revision 6 Spec.
Tiff Class F Specification
VIFF - Used by the Khoros Visualisation package
X - The AVS Image Format
XBM - X BitMap Format
XPM - X PixMap Format
XWD - X Window Dump Format