I really cannot take credit for creating shadows - that's been a part of the texture replacement project since the beginning. Initially it was a separate layer but most people ended up incorporating a shadow directly into a field if they wanted one at all. I got into it because I wanted the challenge of making more realistic textures.
I saw stuff like this,
http://i35.tinypic.com/160tqte.jpg
http://i50.tinypic.com/n32kvk.jpg
http://img576.imageshack.us/img576/9820/ganggreen.png
http://img15.imageshack.us/img15/294/phino.jpg
http://i35.tinypic.com/2ajtd1y.jpg
and thought I could do better.
I do a lot of knocking down of layer intensity, fading in layers to get a more realistic look and using multiple layers to get a certain feel. Most football field graphics are slightly transparent, to allow the yard lines to show through and most have dirt and wear affecting the surface. Also, different grasses have a different look that may also change due to the season.
If someone wanted to purchase art I would be willing to work with them. I would need print sizes but I think the graphics should scale up nicely.