The color choice seems washed out a tad. This isn't necessarily a bad thing just not my preference.

I would suggest that you add some detail to the grass, water and bridge. They seem a tad simple. It's a nice seen, though. Maybe add a reflection of the bridge and grass to the stream or something.

I see your placement and size is good. I like it. I'm far from being an expert, but let me think/type out loud.

You do not need to use outlines, however if you're relatively new to pixel art, you might want to try. If outlines are to be avoided, by all means don't use them.

Your grass looks ok. I suggest looking at some tutorials on this forum. You could also texture the grass behind the bridge, but keep in mind that objects that are farther and farther away loose saturation. This makes the color weak, making it look distant.

Also, I don't sense a light source coming from anywhere. If the sun was hitting the bridge there'd be darker shades in the areas farthest away from the light. Or, shades from shapes that are are casting long shadows. Define your light source.

Also, detail your water more. One of the best ways to emulate it is to find real photos of rivers/streams and try to analyze or reproduce the effects. Light acts differently, depending on the surface and/or how that surface is moving.