The issue is that when I set a featured image for a post is look downgraded in quality. For example, here: http://hellotecnologia.com/blog/internet/las-20-cosas-mas-buscadas-en-internet-wikipedia/. However, if you look at the original image it look way better in quality: http://hellotecnologia.com/blog/wp-content/uploads/2010/11/wikilogo1.jpg.
The original picture size is: 150 * 156.
The featured image size is: 150 * 150.