Custom Tile Problem

[MrCrowley]

Im editing the WhiteMarble tile to look more like concrete sidewalk. Its whenever i edit the tile at all (at texture, even just making it darker) I get these ugly black lines that seperate the tile. All the textures i use are seamless. What could the problem be.

Im using GIMP

 

[Infinitynexus]

It's cause the edgelines always bleed into the next row of pixels on the texture when applied to the terrain.