Why tooltip in main calendar doesn't show the description image with friendly url and yes do it without it?

Each event has a Description with a big image and a little image from Standard Image 1 that I can see in the cell of the main calendar. The tooltip shows Descriptions in this main calendar when the url aren't friendly but if I set the option friendly url in the joomla system control panel, the image isn't showed unless I write a absolute url.
Would be possible the image was showed with a relative rute? It'd evoid I could to rewrite all rutes in the calendar

relative rute without friendly url
http://alhma.com/maquina-virtual/index.php?option=com_jevents&view=month&layout=calendar&Itemid=2374&lang=es

relative rute with friendly url
http://cndanza2.inaem.mcu.es/es/calendario17-18/eventospormes/2018/1/-.html