Hi eschossow,
Up front for you I’m not Joomlart staff or a moderator or support guy. Just another community member happy to help. Means that there is a obviously a risk but if you take a back up of the files you will be changing you should be fine.
I’m happy to point you in the right direction for the css part but as I don’t have Joomla 1.5 (I run 2.5) and I don’t have Opal so don’t know your version of slideshow I won’t be able to help you there.
FYI, re background pictures, you will need to make a decision as to exactly what you want to do there. Have one that repeats itself, have a really big one that is fixed and doesn’t move when a viewer scrolls the site etc. To give you a guide the current top picture you have on the site is 1250×518 so to have one picture it will need to be pretty long!! http://www.thatstoofah.com/joomla/templates/ja_opal/images/body-bg.jpg
Up to you, if I can help, I’m more than happy; if you want to wait for a JA guy, I totally understand!