Tips for a beginner texture artist?

[NerdKoopa]

So, I've wandered into the amazing world of WAD editing and custom textures. I've made some wall textures, and got them to work fine. However, I'm asking this: What makes a good texture / sprite? For example, I don't think you can just add noise to a flat color and call it sand or grass.
I've studied the textures of Doom 2, and noticed that there are very few "clean" textures. Most surfaces, whenever metal or concrete, are a little bit dirty or worn out. Is this because "mint condition" surfaces would have to be drawn with mostly flat colors, making them dull to look at? If I were to draw, let's say, a freshly painted metal wall, would I have to make up for the flat colors with interesting shapes and designs like rivets, vents, logos, trim and pipes?
Also, is it acceptable to digitize an existing image to use as a texture? Like scaling down and tidying up a picture of a pine tree to be used as a sprite?

Here's my attempt at making a bamboo wall. I used Paint.NET and Doom 2 palette that I got from doomwiki.org. http://imageshack.us/photo/my-images/163/bamb03.png/ (The wall is a bit yellow, but that's the closest that the Doom 2 palette can get to dry bamboo.)

Tips and constructive critisism are welcome.

[Memfis]

NerdKoopa said:

Here's my attempt at making a bamboo wall. I used Paint.NET and Doom 2 palette that I got from doomwiki.org. http://imageshack.us/photo/my-images/163/bamb03.png/

It's too cartoonish I think, looks more like something out of a 8-bit game.
This is more like it.

[schwerpunk]

That bamboo wall is amazing. Reminds of something we might see if Wolf3D (it's not brown enough for Doom) had been set in a Japanese POW camp instead of a Nazi stronghold.

Re digitized still images: The Doom creatures were originally based on models, so I don't see the problem. However, you would probably have to palette-adjust them (naturally), and alter them somewhat to fit the theme (unless you were going for a new WAD with other realistic textures, sprites, etc.).

[NerdKoopa]

Ok, I made two more color variations of the wall.
http://imageshack.us/photo/my-images/256/bamboodemo.png/
Which one of the three is best?

[NerdKoopa]

Ok, I've done some more drawing exercises. As it turned out, I can't draw clounds worth crap. This is what happened when I tried to make a bright ocean view with simple clouds.
http://imageshack.us/photo/my-images/32/screenshotdoom201208171.png/

I also practiced drawing static sprites.
http://imageshack.us/photo/my-images/521/screenshotdoom201208171.png/

Do I have any potential at all?