Need PHP example for adding images to posts

I have a site with thousands of posts. I have a folder with hundreds of images in it. Each image goes with one post (not all posts have an image). Rather than upload each image through the UI, I want to write a PHP script to loop through all the images, upload them and associate them with posts. It would create the thumbnail as well. I figure this would be easier than manually uploading hundreds of photos. Does anyone have an example of uploading an image to a post? I can take it from there.

[Bumping after less than one hour! Seriously? No bumping. If it’s that urgent, consider hiring someone.]