I checked out your png and it is completely the way it should be. To make additive blending you use glBlendFunc(GL_SRC_ALPHA,GL_ONE); which is what you did... Also, the balls "blending" effect is exactly what should happen. Try it with different colors, for example red ball and blue ball overlapped should result in purple color in their intersection. Or 10 very dark green balls result in a light green etc...

EDIT: Oh! yeah when it comes to additive blending you can indeed get rid of alpha channel. Images on black background in RGB image do good It will also take less video memory.