You can enable custom icons per virtual machine in phpVirtualBox by adding:
var $enableCustomIcons = true;
to config.php.
Once custom icons are enabled, you may edit a VM's icon by bringing up its settings.
The Icon URL setting may either bet set to a full URL such as:
http://www.virtualbox.org/graphics/vbox_logo2_gradient.png
or a relative URL like:
images/spinner.gif