How to center your header

1. In the Layout tab click on HTML Code

2. Type cntrl F and a little search box will pop up in the browser just above the blogger window. 

3. Type #header img into the search box.  It should pull up this bit of code. 

#header img {
margin-$startSide: auto;
margin-$endSide: auto;
}

4.  Change that bit of code to this bit of code (copy and paste)

#header img {
margin-$startSide: auto;
margin-$endSide: auto;
position: relative;
left: -15px;
}

You can adjust the centering by changing the -15px.  If you make the number smaller (-50) it will move to the left.  If you make the number larger (-5) it will move to the right.

UPDATE:  When trying to center your header this coding only works if you've selected "place instead of Title and Description".  Otherwise it fights other code that exists in your template and won't center the image.