By browsing the internet, my brother Massimo found out the solution for the problem described here. This problem is due to the router.
As explained here:
"When image results are to be displayed, Safari and Firefox make multiple simultaneous connections to the host to retrieve them. This is usually faster than downloading one and moving on to the next and on and on."
"Some consumer-level SPI firewalls misinterpret the attempt to open that many simultaneous connections to one server as a "SYN flood" and block the traffic. Not good, especially when the connections are being made from your machine to an outside host, so the firewall is effectively blocking you from perpetrating what it thinks is a SYN flood."
The SYN flood attack is visible in the router logs. The solution:
"If your router allows configuration of its SPI firewall, you may be able to solve this problem if it has a setting labeled something like:
Maximum incomplete TCP/UDP sessions number from same host
On those routers, this setting is often set to a default of "10"; simply increasing this value to a much higher value - many have had good luck with "20" - will allow accesses to work as desired and will also allow some room for possible future expansion in the number of simultaneous queries made.
If your router does not offer such a setting, there's no solution other than to disable the firewall.
Note that any operating system - Linux, Solaris, even perhaps Windows 7 - could trigger the same problem. You can even generate the same issue in Windows XP by applying "speed tweaks" such as this."