How to automatically resize images
When you install your new theme, you may notice that your images aren’t the correct sizes.
In order to make sure your images are the correct size so that they look perfect, you will need to regenerate these images.
To do this, we recommend using a plugin called Regenerate Thumbnails. This is one of the recommended plugins for your theme.
Once you have installed and activated the plugin, hover over Tools in the left-hand menu of WordPress and click Regenerate Thumbnails.
On the Regenerate Thumbnails page, click the button that says Regenerate Thumbnails For All X Attachments (X being the number of images that need resizing).
Then you just need to wait while the images are regenerated. If you have a lot of images, this may take some time.
Once the images have finished being regenerated, you should see that your images are the correct size and fit perfectly with your theme.