Sage seems to ignore the transparent=True option.
thank you for your help.
x,y,z,t = var("x,y,z,t");
theGraph= implicit_plot3d(z-sqrt(49-x^2-y^2),[x,-10,10],[y,-10,10],[z,-13,13], plot_points=70, smooth=True, color="red", opacity=.98, axes=True, transparent=True);
(theGraph).save(filename="/Volumes/xyz/Users/xyz/Desktop/xxxyyyzzz.png", axes=True, transparent=True, fontsize="12")