You can override the CSS class of alertify.
Just look the example below: http://www.fabien-d.github.io/alertify.js/assets/js/lib/alertify/alertify.bootstrap.css
In your case, you just want to override .alertify
CSS class like this :
.alertify.popup1 {
background: url(path/file1.png);
}
.alertify.popup2 {
background: url(path/file2.png);
}
/* etc */
And use the Javascript like this:
$("alert1").onclick = function () {
alertify.alert("This is an alert dialog", function() {}, 'popup1');
};
$("alert2").onclick = function () {
alertify.alert("This is an other alert dialog", function() {}, 'popup2');
};