I am using 'featured image' fields in my post and the images are sized and compressed already but whenever I upload an image that has some text, I keep noticing the image reduced in quality (specifically, the text in the image). Is wordpress compressing my images? if so, how to prevent it?
A nice explanation is at WPMUdev: http://premium.wpmudev.org/blog/how-to-change-jpeg-compression-in-wordpress/
According to their tutorial, you'd need to add the following to your
add_filter( 'jpeg_quality', create_function( '', 'return 100;' ) );
They also suggest regenerating your thumbnails once the change is made so previously uploaded images will be affected by the quality change.