The important thing that may not be immediatly noticable, is that you can't use anti-aliasing on the outside edge of your transparency map. It has to be: 1 pixel 100% white, the next pixel 100% black. If there's a grey there, you will get a partial transparency, and that may be the color around your image. In the shader, in the blender part, the 'sampling' filter is used instead of 'gaussian' so that it doesn't blur the edge, which would cause the same problem. By the way, this is also a great way to mak a forest of trees. If you have a clean photo of a tree, make a black/white version of it, and set up the same shader. Then take 2 planes, set them at right angles to each other, and assign the shader to them both. At most angles, you'll see a tree with clear areas between the leaves. Also, set the renderer to 'light through transparency" or you'll end up with strange shadows. Hope this helps!