Sorry, I'm a beginner in this forum as well... So first I had a problem with my whole layout after having changed my theme: it appeared all mixed up on the blog - but I could resolve that problem by copypasting the whole CSS-text from one of my other blogs.
Only now I realised that my header-image is still not appearing (even though it appears on the preview) and my undertitle is gone as well. It's shurely a very easy question for you, but for me it's as a matter of fact quite hard to handle... ;-) Thank you!
http://www.plappermaul.wordpress.com