Well, I noticed that most of the UVs are ouf of the image, so that makes me wonder what color are they? I mean, black is transparent and white is opaque, when texturing with NURBS I use the 2DPlacement note to center the image, but that leaves a gray border around it (assuming that I had to resize it down). so then I can go to the file node and change the offset color to black (or to white depending of what I'm trying to do).
But with the UVs in polygons I don't know how that works. I mean, is the area outside the image gray? Why don't you try changing to NURBS and doing it like I do, or try creating a black square (512x512) in photoshop and put the image that you want right in the middle of it and then have all the UVs within the picture. Also, I would separate the alpha and then actual picture. Create two files, one for the color and one for transparency. I usually do that because in the past I've had some truble getting the alpha channel to work right (or I don't have much control over it).