Fuzzy images

I have the latest updates of WP and Responsive, but recently noticed that the header (a graphic of text of my name) and some images are fuzzy, whereas only a while ago they were as crisp as they were when I created the site 3 years ago. Most of the photos and images on my blog and other pages are fine, and it seems that the fuzziness is confined to images in widgets (sidebars, 3-column effect) and the header, which is on every page. I tried replacing them with larger sized images, with images with higher resolution, with sharper text (in Photoshop) images, but nothing works. Those particular ones look out-of-focus.

I had a widget plugin for one particular reason, but since I no longer needed it, I deactivated it. That didn’t solve the problem anyway.

I’m not so technically savvy that I want to mess around with the function.php, but I don’t know what to do.