This feature is available in Contao 4.8 and later.

With this command you can process all deferred images that were not yet resized.

 php vendor/bin/contao-console contao:resize-images [options]
Option Description
--concurrent Run multiple processes concurrently with a value larger than 1 or pause between resizes to limit CPU utilization with values lower than 1.0. On a shared hosting environment it might make sense to use a value lower than 0.5 to limit the CPU consumption to 50%.
--time-limit Time limit in seconds after which the command should stop execution.
--image=IMAGE Pass an image path like 1/foobar-f6eac395d.jpg (without the assets/images prefix) to resize one specific image only.
--no-sub-process Disables the use of a sub process for each image resize. Use with caution, this could result in an extremely high memory consumption.
--preserve-missing Do not delete deferred image references to images that no longer exist.