Infowise People Search allows you to add user search functionality to any site, even with the free version of SharePoint.
Its small size allows the web part to be placed anywhere on the page, such as within top or size navigation bar. You can also easily adjust the web part to your needs, using CSS classes and web part properties.
You can use any field from your Active Directory user profile schema, both built-in and user-defined.
You can specify location and naming convention of user pictures to be displayed in search results.