XWD stands for X Window dump. This file format is used to store and restore screen window images. Two versions of this file exist, the X10 window dump and the X11 window dump. These versions differ slightly in the format of the data (i.e. headers and bits per pixel).
The version X10 XWD file cannot have more than 8 BPP image data.
For the version X10 XWD files, you can read and write the following bits per pixel: 1, 4, 8.
For the version X11 XWD files, you can read and write the following bits per pixel: 1, 4, 8, 16. 24, 32.