All thumbnails in WordPress has default size 150x150px. This sometimes causes that images are blurred. To fix that you should do 2 simple steps.
Set thumbnail size to larger¶
In WordPress admin go to Settings > Media and set Thumbnail size at least to 300x300px and Medium size to 600x600px.
At first install Regenerate Thumbnails plugin from wordpress.org. After successful installation and activation regenerate thumbnails.