If you have photoshop, you could desaturate it completely(Ctrl+Shift+U) and play with curves(Ctrl+U) and/or levels(Ctrl+L), until you get something that’s pretty (conrasty ?). This could work for both, the spec and the bump map.
If you don’t have photoshop, I’d be more than pleased to help you out.
thx Warius, made a few tests, if thats the way you can do a bump map I will try it with Gimp and when not pleasing I will send it to you ;-
ok here is an update
now with a little bit ivy action (http://graphics.uni-konstanz.de/~luft/ivy_generator/), darker stone, better gras which is hard to be seen at that render level, cracks in the glass, a sky, an hdri map and refinded textures.
hope these triangles at the top vanish after a while …
counting on your crits