There are a few different settings which control image width.
First, the limit width setting on the Appearance → Custom Design → CSS page is the width of the main content column in the theme. Themes try to keep images at that width or smaller.
Second, your "#core-content" width setting controls how big the main column actually appears. Of the limit width and that settings, it should use which ever one is smaller.
Third, there are size options in your Settings → Media page control the containing sizes for various image sizes that you choose when you insert an image into a post. If you choose full width or any size option that is set larger than the limit width or CSS-defined width, then the image will fit to the smallest size option of the three.
In looking at your blog http://michellegd.com/ it appears you were already able to figure out how to change part of the theme width but I think it will look better if you copy and paste the example from this reply and change 600px to 800px: