Don’t edit the Twenty Ten theme! Your changes will be over-written the next time you upgrade WordPress or the theme. For this reason, it is recommended that you consider creating a child theme for your customisations.
what code did you use to add the logo?
did you use an absolute image url, or a relative path, which will probably not work?
a link to your site might help to illustrate your issue.
I can’t see any image or image markup in the header of that site.
@ esmi: sorry, here you go, you should see the logo now.
The header markup is different on underlying pages in Twenty Ten. Also your changes will be over-written the next time you upgrade WordPress or the theme. For this reason, it is recommended that you consider creating a child theme for your customisations.