If you notice that some of your images are pixelated or blurry, then you can go to your Media Library, switch to List View, and look for the image(s) that look bad.
On the right side of the image row you'll see these options:
We recommend you to try first to re-optimize the image as Glossy, as usually this fixes the issue. In the very rare cases when this doesn't happen, then you should go for Lossless.