My guess -- the line is in your sprite. You need to bleed the colored edges outwards and ensure that the Alpha (and I do mean Alpha, not Transparency!) is properly clear.
PNGs are a poor choice for source images as all transparent pixels are actually "white" under the hood, as you can see in the image preview you attached. Coincidentally, a white line is exactly what you're seeing. Toggle the Alpha on the Preview window.
Use PSDs whenever possible, TGAs if you can't. Don't use PNGs.
