Unless I'm missing an option for this in preferences somewhere...
When I save a render the file names always comes in as 'Untitled' which I remove and put in usually something based upon the tgd file name, so with valley.tgd, I name renders of valley-1.tiff valley-2.tiff etc it would be nice (IMHO!) if terragen had an option to support a user preference for file naming (and maybe output directory location) based up the name of the tgd file. This would also prevent me from saving tiff files deep in a sub directory tree when I'd recently navigated to add a plant population rather than with the tgd file - or somewhere where I usually put the renders.
Clearly there's possibilities of clobbering an output file but that's always been true and the OS warns you if you're about to do that. As a subset of this (and maybe easier to add) if you're doing incremental saves just giving me the name of the tgd file (without suffix) as the output when saving a render would suit my way of working.