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Final solution (By @GoldenAxe), that is only an example to check that the image fetching is working correctly, of-course it can be done much neatly. In the WelcomeServlet.doGet:

String accessToken = (String) request.getSession().getAttribute(ACCESS_TOKEN);
String apiEndPoint = (String) request.getSession().getAttribute(INSTANCE_URL);
CookieManager cookieManager = new CookieManager();
cookieManager.setCookiePolicy(CookiePolicy.ACCEPT_ALL);
CookieHandler.setDefault(cookieManager);
URL url = null;
try
{
    url = new URL(apiEndPoint + "/secur/frontdoor.jsp?sid=" + accessToken);
    // open's a connection with the url specified and returns URLConnection object
    URLConnection urlConnection = url.openConnection();
    // get's the contents from this url specifies
    urlConnection.getContent();
}
catch (MalformedURLException e)
{
    // TODO Auto-generated catch block
    e.printStackTrace();
}
catch (IOException e)
{
    e.printStackTrace();
}
String sid = "";
// returns the cookie store(bunch of cookies)
CookieStore cookieStore = cookieManager.getCookieStore();
// getting cookies which returns in the form of List of type HttpCookie
List<HttpCookie> listOfcookies = cookieStore.getCookies();
for (HttpCookie httpCookie : listOfcookies)
{
    System.out.println("Cookie Name : " + httpCookie.getName() + " Cookie Value : " + httpCookie.getValue());
    if (httpCookie.getName().compareTo("sid") == 0)
    {
        sid = httpCookie.getValue();
    }
}
String urlString = "https://c.eu0.content.force.com/servlet/rtaImage?eid=ka3200000004MwH&feoid=00N20000008fSIK&refid=0EM20000000TzfR";
HttpClient client = new HttpClient();
GetMethod method = new GetMethod();
method.setRequestHeader("Cookie", "sid=" + sid);
method.setURI(new URI(urlString, true));
int returnCode = client.executeMethod(method);
if (returnCode != HttpStatus.SC_OK)
{
    System.err.println("Unable to fetch image, status code: " + returnCode);
}
InputStream imageData = method.getResponseBodyAsStream();
OutputStream out = new BufferedOutputStream(new FileOutputStream("test-image.jpg"));
for (int i; (i = imageData.read()) != -1;)
{
    out.write(i);
}
imageData.close();
out.close();
method.releaseConnection();

Where did you get your accessToken from? If you used OAuth to establish the session then you will need to have the web OAuth scope to use the sessionid for what is essentially a screen scraping web request. Having the api scope would be sufficient for the Partner API, but for for mocking UI requests.

Other than that, as you have found, you need to put the sessionId/accessToken into a sid cookie along with the request.

You can check your OAuth Scopes under

Setup > App Setup > Create > Apps > [App Name].

It should appear next to Selected OAuth Scopes.

When calling /services/oauth2/authorize I'm using response_type=Token. I'm also passing a scope query string parameter that includes web. So I've specified the web scope on both the App and when initiating the OAuth process.

It appears you are using the Web Server OAuth Authentication Flow. The scope parameter is applicable here as well.

scope
Specifies what data your application can access. See “Scope Parameter Values” in the online help for more information.

Where did you get your accessToken from? If you used OAuth to establish the session then you will need to have the web OAuth scope to use the sessionid for what is essentially a screen scraping web request. Having the api scope would be sufficient for the Partner API, but for for mocking UI requests.

Other than that, as you have found, you need to put the sessionId/accessToken into a sid cookie along with the request.

You can check your OAuth Scopes under

Setup > App Setup > Create > Apps > [App Name].

It should appear next to Selected OAuth Scopes.

When calling /services/oauth2/authorize I'm using response_type=Token. I'm also passing a scope query string parameter that includes web. So I've specified the web scope on both the App and when initiating the OAuth process.

It appears you are using the Web Server OAuth Authentication Flow (based on the response_type=code). The scope parameter is applicable here as well.

scope
Specifies what data your application can access. See “Scope Parameter Values” in the online help for more information.

So you try modifying the authUrl line:

authUrl = environment + "/services/oauth2/authorize?response_type=code&client_id=" + clientId +
 "&redirect_uri=" + URLEncoder.encode(redirectUri, "UTF-8") +
 "&scope=full%20web";

Once you have a sessionId/accessToken, you can verify that it is valid for a web session in a browser using

https://<instanceUrl>/secur/frontdoor.jsp?sid=<accessToken>"

This should bounce you into a browser session with the sid cookie set. If it doesn't work, the session isn't valid for the web, which you will need to screenscrape an image via servlet/rtaImage.

Where did you get your accessToken from? If you used OAuth to establish the session then you will need to have the web OAuth scope to use the sessionid for what is essentially a screen scraping web request. Having the api scope would be sufficient for the Partner API, but for for mocking UI requests.

Other than that, as you have found, you need to put the sessionId/accessToken into a sid cookie along with the request.

Where did you get your accessToken from? If you used OAuth to establish the session then you will need to have the web OAuth scope to use the sessionid for what is essentially a screen scraping web request. Having the api scope would be sufficient for the Partner API, but for for mocking UI requests.

Other than that, as you have found, you need to put the sessionId/accessToken into a sid cookie along with the request.

You can check your OAuth Scopes under

Setup > App Setup > Create > Apps > [App Name].

It should appear next to Selected OAuth Scopes.

When calling /services/oauth2/authorize I'm using response_type=Token. I'm also passing a scope query string parameter that includes web. So I've specified the web scope on both the App and when initiating the OAuth process.

It appears you are using the Web Server OAuth Authentication Flow. The scope parameter is applicable here as well.

scope
Specifies what data your application can access. See “Scope Parameter Values” in the online help for more information.