Image width being condensed even though plenty of space

I’m creating a New Page and selected One Column No Sidebar as the template instead of the default because I want the content to fill the full 940px width of the page.

When I enter test copy and then Preview, the text fills the full 940px width.

When I enter a .jpg that’s 940px wide and then Preview, the image condenses and doesn’t fill the full 940px width of the page. Might be filling 60% of that.

I’m using twentyten. And the <div id> for .one-column #content has the width set to 940px. Not sure why an image would be effected differently than text.

Appreciate any tips of where to look to figure out how to make the images equal in width to the available space. Thanks.