Admin Forum:Changing the symbol of my wiki

In the upper left hand corner of each page of a wiki is the stmbol of it. How do I remove the default one an replace it with a custom one/ Thanks in advance
 * Special:ThemeDesigner&mdash;it's the wordmark. 18:14, January 17, 2012 (UTC)
 * Make sure the image you use is 250 px wide by 64 px tall and a PNG (Portable Network Graphic). Otherwise it might not work. -- Fandyllic (talk &middot; contr) 18 Jan 2012 4:58 PM Pacific