Minecraft won't let me add severs

When I attempt to add a server to my list, (I tried Mineplex) it doesnt register. On screen it says: Can't connect to server. Instead of the Mineplex logo, a black and white landscape is in the thumbnail.
What I expected: I expected Mineplex to load and let me join.
What happened: The Mineplex logo didn't pop up or let me join.