Large Image Plugin?

I’m looking for a plugin that will allow a large image to be used for each post and interact with the theme. This could use custom fields or an inline tag, but would allow for the image to be used as a background image in the theme.

I’m probably not explaining it correctly. But basically I’m looking for a plugin that allows for each post to have a default image that can be used in the post and then elsewhere in the theme. Does something like that exist?