Image Processing Queue is an alternative to on-the-fly (OTF) image processing (e.g. Aqua Resizer)
for WordPress themes.
Like OTF image processing, it allows theme developers to define image sizes for specific theme
contexts rather than defining a size for all uploaded images. This greatly reduces the number of
resized images and hence reduces disk space usage and the wait time when uploading an image.
Image Processing Queue differs from OTF image processing in how it behaves when an image doesn’t
exist yet. OTF generates the image right away and the end-user has to wait for the image to be
generated. With Image Processing Queue there’s no waiting. It immediately returns an image that
already exists (that is the closest fit to the image size requested) and adds the image size to a
queue to be generated in the background as server resources allow. Typically the images are
generated right away and will be available for the next request.
Image Processing Queue also accommodates responsive themes much better than OTF. It allows theme
developers to define a list of image sizes that will work best for their theme’s responsive
breakpoints. Images generated by Image Processing Queue are added to the post meta so that WordPress
core’s responsive functions will automatically add them to the srcset and delete them from the
filesystem when the image is deleted from the Media Library.