If you previously had images on your blog or previously had images that have been uploaded to your Media Library, you will need to run Regenerate Thumbnails plugin so the images can display the correct size for your new theme.
- Install and activate Regenerate Thumbnail plugin.
- Go to Tools > Regenerate thumbnail.
- Click “Regenerate Thumbnails for All Attachments”.
- Wait until it finishes running. Do not close the page yet.
Sometimes some images are still looking weird, check the image size that you uploaded, maybe the size is too small for the new theme. Upload a new larger image for that post (the featured image only).