// Chiamata ajax
function reqImg( request, user, password, index, callback){
    var url = "URL_IMAGE_API";

    var params = "request=" + request +"&user=" + user +"&password=" + password +"&carriera=" + carriera;

    var http = new XMLHttpRequest();

    http.open("POST", url, true);

    //Send the proper header information along with the request
    http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

    http.onreadystatechange = function() {//Call a function when the state changes.
    if(http.readyState == 4 && http.status == 200) {
        callback(http.response);
        }
    }
    http.send(params);
}

////// QUI LO SNIPPET CHE USO PER PRINTARLA
reqImg("imgProfilo", "user", "pass", "0", function( response ) {
    //$('#imgProfilo').append(response); // non funziona
    $('#imgProfilo').append('<img src="data:image/png;base64,' + response + '" />');
});