XPM (which stands for X Window System PixMaps) is a derivation of the XBM format designed to represent color images in a simple to process format. See http://en.wikipedia.org/wiki/X_PixMap for information on the format.
See Img for a Tk extension which can assist in displaying images in xpm format, or XPM to photo image or Tk_Xpm for specialized extensions/procs to handle XPMs.
DKF: One of the key features of the XPM format (and XBM too) is that it is a valid C declaration, which means that embedding them into a C program is just a matter of #include...