allegro_icon man page
allegro_icon — Pointer to the Allegro X11 icon.
extern void *allegro_icon;
This is a pointer to the Allegro X11 icon, which is in the format of standard .xpm bitmap data. You do not normally have to bother with this at all: you can use the xfixicon.sh utility from the tools/x11 directory to convert a true colour bitmap to a C file that you only need to link with your own code to set the icon.