now I got what you're doing. You defined the background in reset.css. Defining it as url(images/grass.jpg) will use the at this time current directory. So it'll end up searching the image in
Ok .. If you still know what you've already changed in the different CSS files, write it down because you need to re-install Thematic .. take a look into your thematic folder. There you should find a folder thematic-sample-child-theme .. Follow the instructions in the readme.txt to create a child theme. Start to work with the child theme. CSS will go into your child theme's style.css .. if you need to change Thematic functionality most of it can be done using the child theme's functions.php.
Let me know, if you need any additional help.