parametric stone wall: problem with image sampler?!

Hi,

I am working on an parametric brick facade. The program works as I want it (maby it is not perfect at it works) but I have problems with loading a picture in the image sampler.

1. If I load a plain picture in the smapler (e.g. black or white or grey) then every stone changes its size like it should. But if I load a pichture frome something else sometimes it happens that partly (not everywhere) just every second row changes?! Why?!

2. I do not understand how I can define where the picture starts and where it ends... I tried clamping it but it doesn work like I hopped.

3. The definition is very slow. Is their any idea to make it faster?

I hope someone can help me.