How to remove unneeded images from Uploads folder

I’ve noticed that the Uploads folder, which contains images for my blog, is now about 500 MB. There are lots of files there that I don’t need. Since WordPress copies not only the original files I add to articles, but also a number of different sized versions of them, that folder is full of unneeded files.

How can I a) cull that folder, getting rid of the images I don’t need. And b), set up WordPress so it doesn’t upload all those different sized images?

Many thanks.