Automatically resizing your images
A lot of WordPress website owners believe that changing the size of an image via the Media Settings page will automatically resize every existing use of that photo in WordPress. However, it does not.
Thumbnails are created when you first upload a photo to WordPress.
When changing themes, it is important to note that you may need to resize these previously uploaded WordPress images, no matter what the thumbnail looks like.
How to easily change the size of your WordPress images
Firstly, head to Media Settings to ensure that your images are the correct size to fit in your theme.
Then you simply install a ‘Regenerate Thumbnails’ plugin that allows you to automatically resize every possibly image on your WordPress website to work correctly with your chosen theme. It certainly is easier than manually editing every photo, am I right?
However, in some circumstances, the ‘Regenerate Thumbnails’ plugin won’t work, which can be due to the image being incredibly large.
These plugins do not like to work with original sized, large images, so if the plugin is not working out for your website, ensure that all thumbnail sizes are the correct size, or worst comes to worst, re-upload the problem images.