Changing Header in Blogger?

I am trying to change the header but I am having trouble, What size does the new image need to be? I made a header the exact same size as it is now but when I tried to use it it was not centered and the old header was still there..

This is my blog: dailysliceoftech.blogspot.com

 

Try checking the option, shrink to fit. I used that and it worked a dream for my images which was too big.

 

That does not work.

 

After upoading the image, check the "Instead of title" or "Behind the title" option. Depends on how your wanting the image to appear is.

 

I tried that, using both options, with and without shrink to fit, it does not center correctly, it will be too the right and the old one still shows, odd..

 

Check this out. Maybe you'll find something there.