I've added the following:
ModPagespeed on
ModPagespeedEnableFilters insert_image_dimensions
However, my image still appears as follows:
<img alt="WordPress" src="wp-admin/images/wordpress-logo.png" height="20px" pagespeed_url_hash="818906537">
No width has been inserted here.
(Page URL https://www.medexpress.co.uk/journals/readme.html)