Without the benefit of seeing your actual site, here's a possible solution to your task of dynamically replacing a MEDIA module image based on a querystring value:

1. Find ID of media module from page source.  This ID will be in the form of:
    
      dnn_ctr{module ID value}_ModuleContent
    
2. Use the media module's FOOTER setting (SETTINGS->ADVANCED SETTINGS->FOOTER) to insert a javascript routine.  The routine will:
    
   (a) Parse querystring to get your client value.
    
   (b) Update media module's IMAGE tag to reset SOURCE attribute based on client ID value.

The MEDIA module does not assign an ID value to the image so you'll have to drill down from the module ID we got in STEP 1.  Exactly how you drill down will depend on the container you're using (it can introduce any number of tables, DIVs, SPANs, etc), but it would go something close to this:  
 
   < script >
      // Get client ID from querystring
     { some script here to parse the querystring and get your client's ID value }

     // Replace media module image with client image
     tempParentElement = document.getElementById('{media module id value}');
     tempImgElements = document.tempParentElement.getElementsByTagName('img');
     for (i=0; i<tempImgElements.length; i++)
     {
        if (tempImgElements[i].src == '{your default media image URL value}')
        {
            tempImgElements[i].src = '{URL to your client's header image based on client ID}';
        }
     }
  < /script >

That's off the top of my head (I have not run/tested the code).  You might want to add some error trapping but that should be pretty close to working as-is. 

-mamlin

Hi. I used your code with my parameters and a few changes, don't know if it works, js is not really my thing,

< script >
      // Get client ID from querystring
    var clientId = window.location.search.substring(1);

     // Replace media module image with client image
     tempParentElement = document.getElementById('dnn_ctr370_ModuleContent');
     tempImgElements = document.tempParentElement.getElementsByTagName('img');
     for (i=0; i<tempImgElements.length; i++)
     {
        if (clientId != '')
        {
            tempImgElements[i].src = '/Portals/0/' + clientId + '.jpg';
        }
     }
  < /script >

But all I got was the code visible on the footer of my header.

http://test.agripartner.com/welzijnswijzer/Welzijnswijzer/tabid/36/Default.aspx