JR Post Image is a light-weight plugin that displays an image or multiple images 'attached' to a post in a SEO friendly way.
No. WordPress generates the thumbnails by default.
Add an custom field to the post named 'jr_nopostimage' with the value 'true' (without quotes). Then the post image won't be showed in the post itself, it will be showed on the index page.
You can use the CSS class
.jr-post-image and style the image. Soon you can also change the css class name in the configuration page.