Well, I was goofing around in Photoshop today, and I wanted to try and make the earth. Without further adue, here it is:
(http://www.2dadventure.com/ags/earthtest.PNG)
I thought it looked okay. Slightly lumpy, but nevertheless I had a good time doing it, and that's what's important.
EDIT: Tried it with a completely different approach (screwed up a little, though)
(http://www.2dadventure.com/ags/earthtest2.PNG)
It doesn't look like it has enough perspective. As you can see from this NASA photo (http://antwrp.gsfc.nasa.gov/apod/image/earth_1_apollo17_big.jpg), there is a inner glow around the earth that gives it a spherical shape. Also, it would help if you got rid of that light blue outline.
The second once is better in that it's not such an irregular shape (though Earth is by no means perfectly round ;)). I think you're using too much smudge/blur/airbrush. Look at satellite photos of earth to see what colours it is - not all green and blue, and learn the basics about form so that things don't look so flat.
(If you want an easy tutorial on planets, try this one: http://gallery.artofgregmartin.com/tuts_arts/making_a_planet.html )
Do you have any 3D programs like Maya, 3DsMax or Lightwave? When I used to do pictures of Earth, I'd create a sphere in a 3D Program, slap on an Earth graphic... position the camera and take a 3-D picture. It doesn't necessarilly look real but it would soot your purposes I'm sure.
There is too much variation in whiteness to greyness in the clouds.
I'm a 3d animator and ive done countless earth and moon shots. My tool of choice (which i also teach) is 3dsmax. http://www.elfpro3d.com/Trutles/elfpro.mpg is a quickly put together company logo for my company. i still need to refine the elf at the end. The audio is currently only in 5.1 surround sound so i left it off of this quick mpg conversion. Most of my earth and moon textures are larger than 10k resolution.
EDIT
ah found an old one i did
http://www.elfpro3d.com/Gallery/Trans.mpg