as I stated before if you have an img tag, HubSpot auto resizes the image to it’s width and height that it will display at.
If you are not using an img tag to display the image, you need to do your due diligence to add the resize image function at the URL.
It’s not a trick it’s officially supported functionality. Yes in some cases you may be able to save a few more bytes doing it manually but it generally does a pretty great job, and you don’t have to worry about some one screwing it up at a later date by changing an image.
On top of all of this, HubSpot also automatically makes all images webp for browsers that support it. They do this server side so the file name will still read as jpg or png allowing their caching to continue to work solidly.