Please tell me the address of your site. I would take a look at it.
Some plugin with a feature like “auto fit images”, “adaptive images” or “responsive images” scales small thumbnails to a greater size of 150 x 100 px. Thus the blurry effect.
I’m having this same issue. It just suddenly started doing this with my last two posts and the images I am using are the same size as all my previous posts.
Ok, I fixed it. LOL I did a bit more digging around. In the Appearance > Widgets, I went to the widget, changed the “size of thumbnail” ratio and checked the box next to “Use aspect ratios of original images?”. It is working great now. 🙂
