Working on a sidescroller

[d13f00l]

Edit: Vertically cropped image, removed second image, mostly placeholder art.

So, specifically, I am working on the backdrop right now.

I'm working on a platformer\rpg, starting to try to clean up the tileset and add a background to the first level.  The only thing in this that I did not draw is the main character sprite, a guy on the SomethingAwful forums drew it for me a while back.

So, I'm drawing assets to the best of my capability, I feel like I'm getting better and maybe someday it will evolve beyond "programmer art" 
For this, I do want to keep a boxy, simple, relatively flat look, realistically that's the extent of my ability and it would look worse if I tried to overwork it to make it look better.

I'm stuck on outlining.  The first screenshot, should I outline all the trees, and both layers of background foliage? 
I'm leaning towards yes, outlining the trees, but the trees in the second row outline with warmer, less contrasty color.  The foliage nor background ground I can't decide.

Is there any rule of thumb that should be applied here?  I don't want the background to be distracting, at all, but I also don't want for it to look washed out or like I'm afraid of contrast or saturation, because that look is no good either.

[d13f00l]

Edit: Huge image removed, updated cropped image above in OP.

I think the background works decently.  Now I need to do something about the ground tiles.

[cels]

- Please don't make us scroll past one million pixels of solid colour. Trim those images 

- I don't think the trees benefit from dark outlines. You can use some toned down outlines. The main issue with the trees right now is that they look flat. The light direction, I guess, is just sunlight from above, so you won't have any light from the sides to show the cylindrical shapes. But if everything deep in the forest is darker, then the sides of the tree trunk should be darker, as they're further away. My only advice is to be consistent. If you only outline certain objects, certain trees, certain rocks, then those will stand out as significant to the player. If they're not supposed to be significant, then it will get confusing. (Not talking about very distant background objects, like mountains and clouds, of course)

- Maybe you could show the roots a bit better. If they're connected to the grassy surface, then you could put some grass between the roots, where there is more soil, so the grass looks more organic and not just like a flat carpet.

- It looks like we can see the forest floor in the background, but since this is a side perspective, that wouldn't happen unless there's a hill in the background. And if it is a hill, then I think the tree roots need to reflect that better, by extending further down vertically.

- It almost never looks good when you're mixing different pixel scales. This piece is no exception. Your pixels are 1x1, 2x2 and even 4x4 some places. Render everything with the same scale.

-  In order to make the objects in the foreground pop and stand out from the background, you need to rely on either high contrast (e.g. bright stuff against dark background, or dark stuff on bright), saturation levels (e.g. strong colours on a washed-out background) or contrasting complementary colours (e.g. everything in the background is different tones of green, while important objects in the foreground are different tones of red). I guess that's the rule of thumb you're asking for, unless I'm very much mistaken.

[d13f00l]

Thanks.  Sincerely appreciated.
To make sure I'm understanding right

It is a valley, yes.

Extend those roots, shade that area, make the grass creep up the trees a bit?

I need to be careful shading the second row of trees, don't want to pillow shade.  Maybe could avoid that by shading the left side of the tree slightly more.

Edit: Also working on changing around the ground tiles



I'll redraw the larger ground tiles per your recommendation of keeping the scaling equal across the board.   
I tried instead to draw some more organic looking ground that could have slopes and such, but I think to do that I'd have to redo everything and it'd change the look and feel of the whole game.

I was thinking I'd start with boxy tiles, with the intention of redrawing them later, but as I get further in I'm realizing that even the level design is kind of seriously tied to the tile set, I'm not going to just be able to myself, or have someone else, redraw a few tiles.  All the levels and backgrounds would have to be done if I wanted the game to look more organic.  I guess it's not a bad thing, I'm just going to have to commit all out to a boxy, stylized look.