Thumbnail size

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.

Regenerate thumbnails

At first install Regenerate Thumbnails plugin from After successful installation and activation regenerate thumbnails.