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It depends on what your theme supports. What theme are you using?
/peter

Twenty ten version 1.3

Don’t edit the Twenty Ten theme. Instead create a child theme for your changes. You can download a basic Twenty Ten child theme from here.

Using a child theme is definitely the way to go. I use them all the time since all you need to maintain is your changes, not the whole theme. For your requirement you will need to override the header.php file to add your logo inside the branding div. To create a child theme you will need to include a style.css file. The one that @esmi provided is good as it pulls in the styles from the parent theme.

A logo is such a common requirement, I’m surprised that the standard themes don’t support it.

/peter