If you use 16x16 tilesize they are not very efficient for now, the grid is also higly visible.
Also the dithering on the stones is a little cheap. You should also avoid working on a white background, if thats the case.
I made a little example for the tiles.

As you can see i used the dirt tiles as the base with the grass is growing on it.
For a proper dirt/grass transition you basically need at least 12 tiles.
4 corners for the "outside"
4 corners for the "inside"
4 edges
Plus one grass and dirt tile of course.
There can be more for each type(corners, edges) to eliminate repetivness.

The same technic can be also used on any other kind of transitions, like water or sand.

I only want to point at vgmuseum, there are tons of screenshots for all kind of systems.
http://www.vgmuseum.com/nes.htm for nes.
And little samson looks indeed very good.

Colored one of adarias lines


@Sherman
haha brilliant!

Happy birthday!

Alles gute zum Geburtstag!