I just visited your site. The image is scaling with the size of the page using CSS (or javascript but I’m reasonably sure it’s just CSS).
In all the excitement of this topic (gripping stuff, a real page turner, I could not put it down) how exactly did you generate that image? The original doesn’t look crisp.
https://www.sergiosalina.com/wp-content/uploads/2013/05/cropped-header500.jpg
The resizing will completely ruin the crispyness of that image. That will never, ever, ever, display crisp when it’s resized in the browser and WordPress’s built in cropper won’t do a good job either. Honestly.
See this image? It’s great.
https://www.sergiosalina.com/wp-content/themes/twentyeleven/images/headers/lanterns.jpg
What I suggest you try doing is create the crispy’est image you can and try replacing that uploaded cropped header with it. Create that new image in something like photoshop and save it as the highest quality jpeg that you can.
If that works (I don’t think it will with any resizing) great. If it doesn’t work please consider hiring someone versed in web design.
On that note (and as this topic has well and truly been exausted) I’m closing this thread.