Thank you @George_Thompson.
Unfortunately, I am supposed to deal with the content of the Geoportal, not with its installation. Some of my colleagues did it. Anyway, If I go under Organization, Settings, Server roles I can see that our server only works as Hosting Server. No Raster Analysis server nor Image hosting server are selected. If I try to switch one of these on the same server that is selected for Hosting Server (which is the only one that I can see in the drop-down menu), it says that it is unable to set the server as Raster Analysis Server/Image hosting server and that the ArcGIS Server is not licensed as an Image Server.
Does this mean that we lack some licenses or we just lack of some pieces of the software?
Thank you for your patience!