what exactly do you want to do? Usually you can configure the sites,
save them as templates and import them back with stsadm.exe - is it
that, what you want to do?
If you want to change the image of just one site then follow this:
1. Click the link on top right which says 'Modify Shared Page'
2. Select 'Modify Shared Web Parts' > 'Site Image'
3. In 'Image Link' field enter the url of the image.
4. click OK
