use css styles:
.entry-content img { max-width: 600px; border:1px solid #AAAAAA; }
credit to NoahY
The border is defined here, in case you link the images (gives thick blue border in IE).
I'm doing this on my site using jQuery on page load and some preprocessing on the backend when the content is posted. I am creating both a thumbnail and a local copy of the image. The thumbs are used by the image slider at the top of each category.
You can just do the resize with pure jQuery. Just use $.css() or $.attr(). The problem is getting the images original dimensions.
Here is an example post with a resized image from a few minutes ago.
example
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