I`m very happy to hear things are going well for you. And I took some time to check out your site, definitely coming along! Congratulations!
The image that you want to replace is called mw_joomla_logo.png and it will be located in your /templates
An easy way to make the switch would be to rename the file as mw_joomla_logo_ORIGINAL.png. Then upload your site`s actual logo and simply title it "mw_joomla_logo.png" This way you don`t actually have to change the reference to the file name within your template.
Also, just fyi, there`s an easy way to figure out the name of any image that you want to replace: while looking at the page in your brower, just right click the image and choose "view background image." Then take a look at the url in your brower, and it`ll be showing you the file name (and where it`s located
Hope this helps,