где то в коде:
var img = new Image();
img.src = "http://myServak.com/foo?name=" + document.frm.username.value;


где то в боди:
<form id=frm... ну дальше понятно, надеюсь.

definition foo
    user = params["name"] - парсим строку запроса.
    user.save - сохраняем в базе
end

img.src = "http://myServak.com/foo?name=vasia"

img.src = "http://myServak.com/images/foo.jpg" - но каким образом - вот что не понимаю :((

img.src = "http://myServak.com/foo?name=vasia"

img.src = "http://myServak.com/images/foo.jpg"

mg.src ="http://myServak.com/images/foo.jpg"

Response.Clear();
Response.ContentType="image/jpeg";
oImage.Save(Response.OutputStream,iFormat);
Response.Flush();

<script type="text/javascript">
  
    var commandUrl="http://demos.javascriptref.com/isuseronline.php?"; 
    var timer=null;
    function sendRPC(url, callback)
    {
        if(currentRequest)
            clearTimeout(timer);
            
        currentRequest=new Image();
        currentRequest.src=url;
        setTimeout(callback,  50 );
    }
    
    function readResponse()
    {
        if(!currentRequest.complete){
            timer=setTimeout(readResponse,  50 );
            return;
        }
        
        if(currentRequest.height== 1 )
            alert("юзер не доступен");
        else
            alert("юзер доступен");
            
        timer=currentRequest=null;
        
    }
    
    function isUserOnline(user)
    {
        var params="user="+user;
        sendRPC(commandUrl+params, readResponse);
        return false;
    }
</script>
<a href="#" onclick="return isUserOnline('deagol');">check</a>

<?php
header ('Location: /images/' . $_GET['name'] . '.jpg');
exit ();

def get_image
    redirect_to "/images/" + params[:name] + ".jpg"
end

Клиент:

var commandUrl="/testing/hello?";

function send_rpc(name){
    var newScript=document.createElement('script');
    newScript.src=commandUrl+"name="+name;
    newScript.type="text/javascript";
    document.body.appendChild(newScript);
}

Сервер:

def hello
    render :text => "alert('"+params[:name]+"! Hello from server!');"
end