Yes, it is possible. And there's even a couple of different approaches you could use:

There's a script for taking screenshots on the Unifycommunity Wiki: TakeScreenshot

The method used to do this is: Application.CaptureScreenshot, [EDIT: Which also has a parameter superSize for high resolution screenshots using an integer multiple of the current screen resolution]

There's also an approach to do [-high resolution-] screenshots [EDIT: with any arbitrary resolution]. I have a script which is based on the script posted in Submitting featured content. This will take a screenshot and store it to disk when you press the key "k" (you could change the key by changing the code). You have to attach the script to a camera for it to work:

using UnityEngine;
using System.Collections;

public class HiResScreenShots : MonoBehaviour {
    public int resWidth = 2550; 
    public int resHeight = 3300;

    private bool takeHiResShot = false;

    public static string ScreenShotName(int width, int height) {
        return string.Format("{0}/screenshots/screen_{1}x{2}_{3}.png", 
                             Application.dataPath, 
                             width, height, 
                             System.DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss"));
    }

    public void TakeHiResShot() {
        takeHiResShot = true;
    }

    void LateUpdate() {
        takeHiResShot |= Input.GetKeyDown("k");
        if (takeHiResShot) {
            RenderTexture rt = new RenderTexture(resWidth, resHeight, 24);
            camera.targetTexture = rt;
            Texture2D screenShot = new Texture2D(resWidth, resHeight, TextureFormat.RGB24, false);
            camera.Render();
            RenderTexture.active = rt;
            screenShot.ReadPixels(new Rect(0, 0, resWidth, resHeight), 0, 0);
            camera.targetTexture = null;
            RenderTexture.active = null; // JC: added to avoid errors
            Destroy(rt);
            byte[] bytes = screenShot.EncodeToPNG();
            string filename = ScreenShotName(resWidth, resHeight);
            System.IO.File.WriteAllBytes(filename, bytes);
            Debug.Log(string.Format("Took screenshot to: {0}", filename));
            takeHiResShot = false;
        }
    }
}

It also shouldn't be too hard to post those screenshots to combine this with the example code in the WWWForm-documentation to post those screenshots to a server.

You could try this:

http://unity3d.com/support/documentation/ScriptReference/WWWForm.html

Which from the link in this Answer: http://answers.unity3d.com/questions/292/is-is-possible-to-output-screenshots-from-unity

Yes. Take a look at JPEG Encoding in Native Unity JavaScript by Matthew Wegner, Flashbang Studios.

1. Create a render texture.
2. Apply it on the camera.
3. Reference the camera in the script
4. Set the render texture as active RenderTexture.active
5. Create a Texture2D to which you dump the current RenderTexture
6. Put back the original RenderTexture.active back. Things to worry about: PNG files store alpha channels as well as any other color information. Sometimes some object might not appear because they simply have that information on it. Most of the time the Texture2D that you need has to be simply of type RGB24, not ARGB32.

Here is a code snippet that should help you:

using UnityEngine;
using System.Collections;
using System;

public class ScreenCapture : MonoBehaviour
{
    public RenderTexture overviewTexture;
    GameObject OVcamera;
    public string path = "";

    void Start()
    {
        OVcamera = GameObject.FindGameObjectWithTag("OverviewCamera");
    }

    void LateUpdate()
    {

        if (Input.GetKeyDown("f9"))
        {
            StartCoroutine(TakeScreenShot());
        }

    }

    // return file name
    string fileName(int width, int height)
    {
       return string.Format("screen_{0}x{1}_{2}.png",
                             width, height,
                             System.DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss"));
    }

    public IEnumerator TakeScreenShot()
    {
        yield return new WaitForEndOfFrame();

        Camera camOV = OVcamera.camera;

        RenderTexture currentRT = RenderTexture.active;

        RenderTexture.active = camOV.targetTexture;
        camOV.Render();
        Texture2D imageOverview = new Texture2D(camOV.targetTexture.width, camOV.targetTexture.height, TextureFormat.RGB24, false);
        imageOverview.ReadPixels(new Rect(0, 0, camOV.targetTexture.width, camOV.targetTexture.height), 0, 0);
        imageOverview.Apply();

        RenderTexture.active = currentRT;


        // Encode texture into PNG
        byte[] bytes = imageOverview.EncodeToPNG();

        // save in memory
        string filename = fileName(Convert.ToInt32(imageOverview.width), Convert.ToInt32(imageOverview.height));
        path = Application.persistentDataPath + "/Snapshots/" + filename;

        System.IO.File.WriteAllBytes(path, bytes);
    }
}

Jashan code works when you play in editor, however when build and play the screen shot image was blank , was is due to render target has been changed to 'RenderTexture'?? however i tried just saving the texture without render target texture then it worked fine..:) Thank you so much for your suggestions above..:)