Adding Your Custom Logo To a Site
It is very easy to replace the top left (orange) default SharePoint logo in a site by your own. Just prepare a small image in .jpg, .gif or .png format with reasonable dimensions (the default icon is 32×32) and upload it to a library where all site users have, at least, read permissions. For a top site in a site collection, that could be the Style Library which already has an Images folder. In sub-sites, you can either use the Site Assets or Site Pages or any existing or new library with Read permission for all. Make a note of the image URL (right-click and copy shortcut or mouse over and note the status bar).
To use the logo, go to Site Actions, Site Settings and then click the Title, Description and Icon (under Look and Feel). In the Logo URL section, write the relative image URL (see the partial screen shot). Optionally, you can use the other box for use as an ALT for the image.
If you leave the URL box empty, SharePoint will use the logo from the parent site or revert to the default icon if you are in the top site. And if you add a web part page, the custom logo may not show. In this case, click the Page tab (ribbon) and select Title Bar Properties. A web part opens in edit mode and you can use the same relative image URL in the Image Link box of the web part configuration.
There are other ways of changing the logo including the use of CSS and editing master page. The icon that shows up on the browser tab and to the left of the site URL is based on the favicon.ico file (like internet sites). You can replace this file by your own (must be farm admin and need to access the SharePoint root folder and browse to Templates\images\ which will affect all farm sites, or by editing (a copy of) the master page and adjusting the path of the favicon.ico file (look for the tag “SharePoint:SPShortcutIcon”). This approach is preferred but requires SharePoint Designer 2010.