Yeah, the thumb-nailer built into the board software doesn't handle .PSDs very well.

I've been doing some experimentation with forests and had some pretty good results with the following steps;

1) New Layer
2) Set foreground/background colors to shades of green (or whatever is appropriate)
3) Filter > Render > Clouds
4) Filter > Render > Difference Clouds (at least x2 - but use even number)
5) Filter > Brush Strokes > Ink Outline
6) Select > Color Range {darkest shade(s)} and remove [optional]
7) Filter > Sketch > Bas Relief
Filter > Blur > Gaussian Blur (adjust to taste) [optional]
9) Filter > Noise > Add Noise (Mono & Gaussian - adjust to taste) [optional]
10) Apply pattern overlay of choice (usually with adjustment to Soft Light or Overlay)
11) Apply Layer Mask > Fill With Black
12) Use soft/brush eraser on the mask to reveal the forests where desired

You may want to repeat the process changing the colors to "break the color monotony" of the larger forests.