ofImage Transparency problem

I made some PNG image has a small circle in the center with a transparent background and made it rotate round the Geodesic, but it strange when the structure rotating, the edge of the image which should be transparent begin to cover others. As you can see on the image below, I have marked them with red arrow. Anyone have any idea about that? Many thanks!

transparency is a lot more complicated in openGL that it might seem at first.
check out this article on whats going on here:

http://www.opengl.org/wiki/Transparency-Sorting

Thanks a lot, I will try it