could you post a description of what you are trying to do with the first rule? It looks like you want all requests for files of those extentions to be redirected to /index.html?
If so I would suggest something like this:
RewriteRule ^.*\.(png|jpg|css)$ /index.html
ripped down it goes like this:
^ -> begin pattern match
.* -> '.' represents any single character. the '*' 0 or N of the preceding text (N > 0)
\. -> the backslash escapes the special char '.' so this will match a literal period (.)
(png|jpg|css) -> you seem to have that figured
$ -> end pattern match
so that statment will match strings that have any number of characters that end in either '.png' or '.jpg' or '.css'.
Test this out for yourself, im just learning as well. Most of this info came off apache's own docs.
http://httpd.apache.org/docs/mod/mod_rewrite.html
Good Luck
Aaron