The size of the image helps in maintaining the speed of the site. Blurry images are shown just to shorten the loading time of the site.
If you are facing blurry images in the slider and other widgets section you could just follow the steps below:
1. You need to install the plugin Regenerate Thumbnails
2. Active the plugin
3. Click on the Tool shown on the Dashboard
4. Select the Regenerate Thumbnail Option
5. Then, click regenerate for all attachment
Once you regenerate the thumbnail all the images could be seen in their original form.
You could also manage the image size follow the steps below
Dashboard > Setting > Media > Change the image size and save the change
( Note: You must regenerate the thumbnail once after you change the image size )