Apache Rewrite module, complex rules - .htaccess

Question

I basically want to redirect all requests to index.php doesn't matter what, except those with certain REQUEST_URI. Those requests that look like image files, so have an ending like: .jpg or .png should be examined and if they are under the public/ folder (or any subfolders in any depth) and if they are they should be served and the rewriting process should stop here! If not, I want to redirect to a default image at public/errors/image-not-found.png and terminate rewriting process. The other exceptions are files that end with .js, .css, .html or .swf. They also should only be served if they are located under the public/ folder or any other subfolders. If not, a simple 404-Not found should be sent back. In either case of the last to the rewriting process need to stop of course.
Any other request should be redirected to index.php and appended as a query string. (even if the request points to a directory or to a file that is not under the conditions aforesaid, but exists, e.g: www.xyz.com/library/Database.php -> www.xyz.com/index.php?url=library/Database.php)
I have half-measure solution:This is how I redirect everything to index.php:
RewriteRule ^(.+)$ index.php?url=$1 [QSA,L]
I append a visual explanation of what I want. Maybe this is clearer:

Answer 1

Basically, you don't want to do anything if the requested file exists in public/ or any of its subfolders. So, first we deal with those:
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/public/.*\.(html|css|js|swf|jpe?g|png|gif|bmp|ico)$ [NC]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]
Now, that is over with. We now check whether an image was requested:
RewriteCond %{REQUEST_URI} \.(jpe?g|png|gif|bmp|ico)$ [NC]
RewriteRule ^ /public/errors/image-not-found.png [R,L]
Similarly for other static files:
RewriteCond %{REQUEST_URI} \.(html|css|js|swf)$ [NC]
RewriteRule ^ - [R=404,L]
Redirect everything now to index.php:
RewriteCond %{REQUEST_URI} !/index\.php$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f [NC]
RewriteCond %{REQUEST_FILENAME} !-d [NC]
RewriteRule ^.*$ /index.php?url=$0 [R,L]

Answer 2

Following series of rules should probably mimic the flow-chart:
# for public folder pass through
RewriteRule ^public/.+?\.(?:jpe?g|ico|png|bmp|css|js|html|swf)$ - [L,NC]
# for other images
RewriteRule ^.+?\.(?:jpe?g|ico|png|bmp)$ /public/errors/img-not-found.jpg [L,NC,R=302]
# for other css|js|html|swf URIs
RewriteRule ^.+?\.(?:css|js|html|swf)$ - [L,NC,R=404]
# everything else, route to index.php
RewriteRule ^((?!index\.php).+)$ index.php?url=$1 [NC,QSA,L]