..it depends on the structure of the theme you've installed.. if the developer has put an editable area in the header, you can easy change the image, but i think you would allready have recognized this..

if the image is a part of the theme, you have to look into the themes folder located under /themes/nameofyourtheme

..the most styling information is done in the files wich end with .css ..the image is probably set as a background definition of the header element (something like background: url(imagename.jpg);)

the image could also be placed directly in files ending with .php .. in this case written like <img src="imagename.jpg" />

in both cases you can upload your own image to the themes folder and replace the part in the .css or .php file..

..it's not so easy to explain in a few sentences but if you put your url here, i could have a look how the theme is build..

Shaun,
Why don't you tell us what theme your using as well. It's really easy to edit the .php files to make that header element editable in Concrete5 so you could change the picture, add a slideshow or whatever...

..agree, replacing the image with an editable area would be the best solution..