After you have changed your old theme to this theme. The theme is using different image ratios and/or image dimensions for the thumbnails than your previous WordPress theme was doing. Maybe your images are not being displayed consistently in size on your website.
In this tutorial, we have shared a plugin called "Regenerate Thumbnails", which allows you to regenerate the new thumbnails after changing your WordPress Theme.
Why do you need to regenerate thumbnails after a theme change?
When you upload an image, WordPress will generate thumbnails that are needed by your WordPress theme. After you have changed the theme, these previously generated thumbnails won’t be regenerated automatically. To fix this issue, you can download and install the "Regenerate Thumbnails" plugin and run it once to enforce WordPress to regenerate your thumbnails.
Navigate to Tools -> Regen. Thumbnails and click on Regenerate All thumbnails button.
Regenerate thumbnails via The Library Page
You can also Regenerate thumbnails in bulk as shown below under Media -> Library Page. Select all images for which you want to regenerate the thumbnails and choose "Regenerate thumbnails" in the drop-down, click on Apply button.
What if regenerate your thumbnails do not help?
If you still have issues with image size after regenerating your thumbnails, maybe this due to the reasons below:
Uploaded images don’t meet the minimum requirements of the theme
Check if the uploaded images meet the minimum requirements of the theme. Because in case the uploaded images are too small, regenerating thumbnails usually won’t help as that process won’t enlarge your images and it will skip those images.
The recommended minimum image size to upload is at least 1200*1200px. If you upload images without at least that size, WordPress will not be able to generate all thumbnails as expected.
Clear your cache plugins
Ensure that you have clear all the image cache before checking your website.