Anyone know how to add an additional folder into a URL? For example: www.mysite.com/pine.html www.mysite.com/oak.html changed to: www.mysite.com/wood/pine.html www.mysite.com/wood/oak.html using mod_rewrite / .htaccess? Is it even possible?
Unless your example urls are already obtained with mod rewrite, I'm somewhat puzzled why you want to use mod rewrite for this. Just create a directory called wood in your website's root directory and put your pages pine.html and oak.html in that new directory and you will have the url that you want.
If your pages pine.html and oak.html AND the images/pictures shown on these pages are now in the root directory of your website then move the images/pictures together with the pages into your new wood directory and they will display. Equally if there are other "things" that show on these pages, then move them as well.
On some webservers there are more than one URL for a resource. Usually there are canonical URLs (which should be actually used and distributed) and those which are just shortcuts, internal ones, etc. Independent of which URL the user supplied with the request he should finally see the canonical one only.