After decoding a GRF graphics file with GRF Wizard and the GRFCodec, you’ll have an NFO (plain text) file, and one or more PCX or PNG image files. By editing the NFO file, you can alter the size and position from indivial game sprites (graphics). This is an almost impossible process to do by hand, while NFO Editor makes this a quick and effortless task.
TTD Urban Renewal was a site hosting GRF graphic enhancements created by Andrés for the simulation game Transport Tycoon Deluxe. Since no more GRF files have been released there for a few years, that site was shutdown, but here you can find and download all those graphics and apply them to your TTDPatch or OpenTTD game.
Some of these GRF files are available for direct download from OpenTTD’s Online Content.