You can achieve this by adding JavaScript to you html, Add the following script
<script type="text/javascript">
    (function() {
        var classes = [];
        dojo.connect(mx.ui, "startup", function() {
            dojo.addOnLoad(function() {
                var roles = mx.session.getUserRoles("Name");
                for (var i = 0, role; role = roles[i++];) {
                    var claz = "role-" + role.toLowerCase();
                    dojo.addClass(document.body, claz);
                    classes.push(claz);
                }
            });
        });
        dojo.connect(mx, "logout", function() {
            for (var i = 0, claz; claz = classes.pop();) {
                dojo.removeClass(document.body, claz);
            }
        });
    })();
</script>
below
<script type="text/javascript" src="mxclientsystem/mendix/mendix.js"></script>
in your html.
This JavaScript is to change the logo by userRole.
Change the logoPane
.role-administrator .MxClient_logoPane
{
    background-image: url(your image);
}
in your layout.css and change the background image for your different userRoles.