Hi,
If you’ve done a lot of updates, I would definitely suggest backing up your work before updating the theme. It’s very hard to say how difficult this is without knowing exactly what you changed.
Let’s say you only changed CSS. To me, that’s fairly easy. You just back up the modified files, update the theme automatically, and then incorporate your changes into the new version. Sometimes you won’t want to simply copy your changed file over the new file, as there may be new theme changes in the CSS. I do code compares to make sure that my changes are clearly visible, and that everything will work. If that’s not something you’re familiar with, I would suggest at least very clearly marking your changes before backing up and upgrading.
As long as your new background images are not named the same as the originals, they should be safe. You’ll just need to make sure you add you re-add your links to them in CSS or wherever they may be.
If the database is involved, as it is with some complex themes, then it can be tricky. Atahualpa, a very fancy complex theme, will do a great job of keeping your changes – it saves them in the database, and it kept them even when I wanted to just get the theme back to “Atahualpa default” condition. I had to go into the table and delete some stuff in there to do that. So that theme was very good, even too good, at keeping changes. Others will not be. I would suggest looking at the theme author’s site to find out what’s involved in doing a safe upgrade. Possibly backing up those database tables for the theme may be prudent before upgrading.
So many WP newbies have the deadly combination of being careless and not being detail-oriented. They will just do any upgrade immediately without any concern for plugins, themes, or any of that. Then they’re mortified when everything gets all messed up. It’s sad. I can see that you are cautious enough to care, and that’s great.
