Use a spray can with Opacity and Density set at 100.  Place nozzle at the center of where you want the sphere and quickly press the mouse button.  The quicker the touch, the more pronounced the 3D effect. 

This can also be used with two colours (one set for each mouse button).  Providing you don't move the mouse between clicks.

Fine tuning can be carried out using the BRIGHTNESS/CONTRAST ratios and different colours are whatever you select.

Size is done by re-sizing the original image (quicker to display and uses less memory to store it).