[twentyten-child] php file in child

G’day,
I’m using wordpress 3.1.3 with the twentyten theme see https://build.adventureriders.com.au/.

I want to edit header.php to facilitate image rotation in the header. The code would be approx:

[code]
<img src="path_to_images/top_image_<?php echo(rand(1,5)); ?>.jpg"
width="image_width" height="image_height" alt="image_alt_text" />
[/code]

I’ll tidy that up to use already created variables, etc.

My question is:
Do I need to have the entire header.php file? As I cannot see how else it would work. If I do not need to recreate the entire file please explain fully.

Thanking you in anticipation,
build