Images Showing Up Blurry on Theme

This can be for two reasons, the most common is that it is not pulling the full size image.  To fix this, click on the image, then over on the right in the editor, usually under the General tab is a spot that says Thumbnail Image Sizes.  From that dropdown make sure it is selecting the full size.   That will load the full resolution of the image, vs a smaller one. 
Another reason is you have a plugin installed that is changing the quality so the images load faster on the front end.  Depending on the plugin you should be able to adjust the settings.