Featured images only showing in Firefox and Safari

I am displaying featured images on the homepage of my site
http://kondwani.com/stamp/ (see images listed under ‘News’ & ‘Blog’).

The featured images are displaying fine on the homepage in Firefox and Safari, but in Opera, Chrome and IE the featured images are not visible.

I have set up the Featured Images by adding the following to the functions.php.

// Add support for Featured Images
if (function_exists('add_theme_support')) {
add_theme_support('post-thumbnails');
add_image_size('blog', 590, 125, true);

Thumbnails for blog category are 590px by 125px, the thumbnails for news category I’ve set in Media Settings > Thumbnail size (225px by 150px).

Any ideas why they are not visible in Opera, Chrome and IE?

Thanks for any feedback,
Martin White