I seem to be having this issue with all of the images on my site.
First I optimise a .jpg in Photoshop to approx 100k (it's a large header image), then I upload through Wordpress' media uploader as a Featured Image. Here's the code I use to output the image in my template file:
<?php if (has_post_thumbnail( $post->ID )): ?> <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' ); ?> <img src="<?php bloginfo('template_directory'); ?>/thumbs.php?src=<?php echo $image; ?>&w=660&h=246&zc=1" alt="<?php the_title(); ?>" /> <?php endif; ?>
When I view the image in the browser, it seems to have a file size of approx 900k. 9 times the size of the original file!
What is causing this increase in file size after uploading through Wordpress? Is it WP's media uploader or the timthumb.php script?