just spitting in the wind here .(i don't know jack about these things)

I guess the brush you used , have softened/smoothly-blended edges so the parts on the edges are blending through to transparency and the shader cannot pick it up as part of the filtering because the alpha or colour values at those edges aren't "pure" , second possibility could be that antializing is causing the smooth blend at the edges , last possibility could be a bilinear filtering artifact (highly doubt it though) .

and i believe it needs to be 24 bit format.