For Further, the featured images should be at least 672px in width. The grey lines you see are part of the theme. Your top post has an image that is at least that wide. The Danny Kaye post image is not that wide, so the grey patterned background shows. The featured image is set with a touch of transparency and then the pattern shows through just a little when you hover over the image. You can see this more clearly on the top post on the demo site at http://furtherdemo.wordpress.com/ . The easiest way to take care of this is to make certain your featured images are at least 672px in width.
From the Further theme information page:
To get the most of this theme, setting Featured Images is important.
The standard format article displays them beautifully to grab your readers attention. The featured image works best with images at least 672px wide.
You can get rid of the patterned background with CSS by doing the following (if you have the Custom Design Upgrade). The thing is, this will still leave you with some funkiness if you don't use the correct width images.
background: #FFFFFF !important;