alpha blending

hello I don’t understand how blending works, i have set :

 
    gl.disable(gl.DEPTH_TEST);
    gl.enable(gl.BLEND);
    gl.blendFunc(gl.SRC_ALPHA,gl.ONE_MINUS_SRC_ALPHA);
    gl.blendEquation(gl.FUNC_ADD);

And I get this result :
https://img15.hostingpics.net/pics/981897problem.png

could someone explain me what is happening please ?
thanks in advance.

I get what was wrong, gimp save images as premutiplied alpha. I need to use

  this.gl.blendFunc(this.gl.ONE, this.gl.ONE_MINUS_SRC_ALPHA);

Solved.