The text in the menu is not an image, it’s generated through Cufón
. However, since most modern browsers support @font-face nowadays, scripts like these aren’t really necessary anymore. As this appears to be a Wordpress plugin the only thing you need to do is disable/remove it from within the admin area.
Looking at the source code it looks like this is a fairly intricate theme with a lot of sub directories and includes and stuff. The best way to edit such a theme is downloading the entire theme directory, looking at the output source code in the browser for the element you want to edit, and doing a multi-file search (any decent code editor program can do this) for the element. This way you will easily understand the structure of the theme.
However, depending on the complexity of the theme there might be things that are specified in the admin area, not in the theme files. For example, if the theme has a dynamic menu the menu items are also stored in the database, not as actual HTML in a file, so that is edited in the “menus” in the admin area. Look if the theme has specific settings.