By default, WordPress doesn’t create thumbnails on the fly. Custom Image Sizes is a WordPress plugin by Austin Matzko that generates the correctly-sized version of an image’s thumbnail, if it doesn’t already exist, it lets you specify custom sizes with a size parameter of ‘[width]x[height]’
You can tell WordPress to create a particularly-sized thumbnail by registering it with the core add_image_size() function. However, if you don’t want to have to do that, Custom Image Sizes lets you specify a custom size.
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