1.Log in to your dashboard--> layout- -> Edit HTML

2.Scroll down to where you see header-wrapper code block.It will nearly look like this:

#header-wrapper {
width:800px;
margin:0 auto 10px;
border:1px solid $bordercolor;
}

Note :- width:800px can be different value.

3.Now add "display:none;" into your header-wrapper code block.Look at below:

#header-wrapper {
width:800px;
margin:0 auto 10px;
border:1px solid $bordercolor;
display:none;
}

Done. above code hides your blog header.

4.To use a image instead header,find the below code.

<div id='header-wrapper'>
<b:section class='header' id='header' maxwidgets='1' showaddelement='no'>
<b:widget id='Header1' locked='true' title='your header title' type='Header'/>
</b:section>
</div>

5.Now paste the below code just after the above code.

<a href="URL_OF_YOUR_BLOG"><img src="URL_OF_THE_IMAGE" alt="Home" /></a>

Note : Replace URL_OF_YOUR_BLOG and URL_OF_THE_IMAGE with your content.

6.Click on "Save Templates" and you are done.