After posting some screenshots, and feedback from the forum members, and after removing the worst tileability effects I think it is time to do the next step:
Test it in the game !
Because in my opinion that is the best way to see how it looks. So nothing is final yet, any feedback is still appreciated. When the 256px tiles full zoom is implemented, we'll have to see how they combine with Ben's set.
I have created a set of 19 32bpp grass tiles for use in the temperate climate.
They are already PNGCODEC'd
UPDATE:
Because the tarloader is now present in trunk, installation is different from version 1 !
This make installation even easier

-If you've used version 1 of the grass set, remove sprites 3981.png..3999.png from the directory bin/data/sprites/trg1r. Because the untarred png have higher priority over the tar loader, if you don't do this, you'll see no difference with the previous set.
-Check if you have tar files that are not intended for the newest tarloader in the data directory or it's subdirectories. On Linux, from the data directory: find . -name '*.tar' , on Windows do a file search. If the tarloader encounters tar files in the wrong format, it'll give an assertion, so move tars not meant for the new tarloader outside the data directory
-Check if you have a nightly build >=11119, otherwise the new tar loader is not present.
So far the preliminary actions, now for the installation:
copy the grass2.tar file into the bin/data directory.
That's it! Ready.
start openttd as usual with a 32 bpp blitter ( you'll need a nightly build, you can start a 32 blitter with the -b option on the command line (check present blitters with openttd --help ) or specifying it in the config file: blitter = xxx )
If you have the new terrain grf (8bpp) active, I guess you'll have to disable it.
Have fun and let me know what you think.
edit: trees in screenshot are SAC's stolen trees, some road tiles are 32 bpp tiles of Ben Robbins