You have to connect_after or override the signal "draw-background", and paint there what you want as canvas background, such as tiled pixmap, grid, etc. Here's an example for setting tiled pixmap:

 import gtk
 from gtk import gdk
 import gnomecanvas as canvas

 def drawbk(acv, dw, x, y, ww, hh, pb):
     gc = dw.new_gc()
     pw = pb.get_width()
     ph = pb.get_height()
     offset_x = x % pw
     offset_y = y % ph
     dw_y = -offset_y
     while dw_y < hh:
         dw_x = -offset_x
         while dw_x < ww:
             dw.draw_pixbuf(gc, pb, 0, 0, dw_x, dw_y)
             dw_x += pw
         dw_y += ph

 # window 
 win = gtk.Window()
 win.resize(100, 200)
 win.connect('destroy', gtk.main_quit)
 win.realize()

 # load a graphic
 pb = gdk.pixbuf_new_from_file('baize.png')
 w, h = pb.get_width(), pb.get_height()

 cv = canvas.Canvas()
 cv.connect_after('draw-background', drawbk, pb)

 win.add(cv)
 win.show_all()
 gtk.main()