Textures

I have been messing around creating some textures for runways and taxiways for a small airfield. I have used FSSC to create the various elements, however what I cannot get my head around isthe followingIf I create say a shale taxiway, the centre part of the bitmap is the shale and the outer edges are the grass as I want it to blend in. When I assign the texture to the runway I don't get the 'shale' part running up the centre end the grass running the length of the taxiway I get segments of the shale then the grass, then shale and so on.The .bmp is created in photoshop and I usually make it into a 256x256 image.I thought it was me then I tried one off a commercially available piece of software and the same thing happenedAnyone got an Idea what I am doing wrong