I think you might want what "xsetroot" does. There are also variations that use XPM files to set a pattern. Note that having a full-color pixmap is very expensive in terms of server resources.

...but "xsetroot -bitmap" only works with XBM file, doesn`t work with XPM file.

Yes; the "variations" I referred to include xsetrootxpm, which uses XPM or XBM files.

thanks for your help...but i couldn`t find xsetrootxpm. could you please give more details? thanks

At one point, it was with the XPM distribution. See the package for libXpm. It is probably on ftp.x.org and avahi.inria.fr.