Using the same Density map, here are more results:

Setting the density to 50% will make white only have a 50 chance of drawing. Here again with no jitter:
2-nojitter_50percent.png

And back at 100%, but with a jitter of 1. The jitter value is a scale of the spacing parameter, for randomly displacing the x&y directions, to get rid of the grid look:
3-jitter.png

The scrip draws left to right and top to bottom, to work with isometric image brushes, like the trees posted:
4-trees.png

And using the "invert" check will invert the map (black = 100% chance and white = 0% chance):
5-trees_invert_map.png

(cont...)