Kevin already gave you the link about our SiteLogo control, please read the documentation.
If you are making a custom skin you can put a SiteLogo control in your layout.master file and drop your logo file(s) into the /Data/Sites/[SiteID]/logos folder
<portal:SiteLogo id="SiteLogo" runat="server"></portal:SiteLogo>
typically you either use that or:
<portal:SiteTitle id="SiteTitle" runat="server"></portal:SiteTitle>
See jsavard-mojoportal for an example skin that uses it SiteLogo
Note that whether using an image or text or both, you really want an h1 for your site title not just a div or span with style applied.
Css applied has to be specific to the design and the logo.
Hope it helps,