When & How to generate new thumbnails or image sizes in WordPress
By default, when you upload an image, Wordpress will automatically create different sizes and saves them in the uploads folder.
These default image sizes can be easily changed under Settings => Media => Image sizes:
But here's the thing ... Many themes (in fact, most of them) will take advantage of the above feature in order to define new theme-specific image sizes ...
Which is also cool ... until you'll decide to change your theme! The problem? Most likely, the new theme will define new image sizes ... Which new image sizes are only applied on the images you upload after activating the new theme! As a result, all the old thumbnails will look ugly and distorted. Believe me, it's a very common scenario ...
Luckily, the solution is quite simple: you'll have to generate new sizes for all the previously uploaded images ... This process is often referred to as regenerating thumbnails, and it's easier than you think.
So, I've decided to create a short tutorial where I will show you how easy is to regenerate thumbnails or new image sizes in Wordpress.
Here it is: