Well, the search bar has the class ".search"
.searchtext, .searchop, .searchsubmit
background-image: url(URL HERE);
background-position: top left;
The first part will hopefully get rid of the search input, dropdown menu, and the "Search" button. In the second part, replace URL HERE with the URL of the image you want, and you can change the repeat and position of the image as necessary, as well as the background color. Oh, and you can edit the width and height of the bar by replacing the ###s with actual numbers.
I haven't tried this code out; I just studied the default Xanga source code, so if it doesn't work, tell me and I'll see what I can do.