You didn't say which version of WordPress. If it's 2.5, view the source of a page and you'll see the images already have a few classes. One looks like "wp-image-nn" where nn is (guessing here...) the id of the image in your posts table. So you don't have to do a thing! Unless you wanted them to have a different naming format.
Take a peek at the posts table in your database, you'll find that images uploaded into a post are listed in there, they just have a different type. You could write a query to get them, or poke around in the wp-includes directory for the code that does this to display them in your posts.
BUT, that said, since you mentioned css ids, I am assuming you'll start by displaying them all in your post in the HTML.
So, if you just want to post a batch of images and then on the client side hide all except one and rotate through with JavaScript, you CAN select them with jQuery fairly easily. And manipulate them. Without individual ids. Check it out before you write a lot of JavaScript.