Header image and size of post

Hi people, I have two questions. This is my webisite: http://sec.org.rs/ Is there a possibility to change a header in this theme? I want to put image here, instead of logo and text: Also, I would like to have more, but smaller size of posts on index page. Something like this (look image). Is there a way to do it? Thanks in advance. Im so sorry if somebody asks these questions before. I was looking for answers, but didn't find them.