This will happen if your image is very large and you are trying to place it somewhere that is expecting a smaller image. If you have a very large image, when a thumbnail is produced and the image has to be shrunk, sometimes it will turn out blurry, even if the file is high-quality. To fix this, resize the image in an external image editor and then re-upload the smaller image (generally thumbnails will resize the image to 150px square). If you need to use the larger image somewhere else on your site, upload another (smaller) copy of the image specifically for this purpose