Bill, here is a method that breaks down the approach, hopefully into bits that are easier to grasp.

All your master hi-res images are in a Masters folder.
You drag an image from Masters onto your design page and it renders as normal.
What you have created is a replica of the Master image as you probably adjusted its size to fit the page.
It will be given a numeric filename unless you apply its Image Filename.
At the same time your Master image is now part of your design file and occupies a large part of the design file size.
Yes you can Optimise Images but unless you do have a Masters folder and subsequent changes will max out as this images 96/192dpi for its fitted dimensions.

Instead, you create a box the size of what you want the image to appear as on the design page.
You then use its Web Properties > Placeholder > Replace with graphic file and Browse to the file in the Masters folder.
What you now have, when you save your design, is an external assets folder containing the master image, full size.
Nothing is added to the Bitmap gallery unless you have ticked the Regenerate option. This makes a scaled replica of your master image, which can still add into the the design file storage.

Undaunted, you realise that you can use a free tool to create thumbnails of your master images and create a sub-folder to hold them.
Assume they have the same image filename or something very similar for easy reference.
You can now drag the thumbnail and resize it to what you want.
You have no calculations to perform but you have to link to your masters somehow.
A browser will not use a local filestore link as this is a security risk.

Luckily, the Replace with graphic file option can be put to use.
This time use it with any shape on the design page for any graphic file at all.
Ensure Regenerate is unticked to avoid any addition into the Bitmap gallery.
Now locate the newly created external assets folder where this image has been copied.
Delete that and instead copy in all the master images you expect to use.

Here comes the sleight of hand.
For each resized and placed image in your design, open its Placeholder Body and insert:
<img id="unique image identifier" src="<source path>/<A nice image filename>.jpg" alt="describe the image" loading="lazy" width=100% height=100% />
Change the id, set the src to 'index_html_files/<master-image>.<ext>, describe the image in alt.

For your Hero image, remove the loading="lazy" parameter.
This allows Lighthouse and Insights to be happy to find an image early on.
Do the same for any image within the height of the expected viewport, i.e., before the client scrolls down.

You can use Xara to create the thumbnail.
Drop a master image onto the design. Yes, mega size alert.
Clone it and make the clone very small.
Create a Bitmap Copy of this small clone. try 64px for its longest side.
Resize and fit to fully cover the original.
Delete the original.
When you save and re-open the design file, the original image will have disappeared from the Bitmap gallery, as any unused bitmaps (nowhere on any page or layer) will be deleted from the file.

A long way round!
What you are trying to achieve is all you large images to end up as external assets.
You can then drag it onto the design, and make a very small thumbnail copy.
Then you can Replace with graphic file and rescale over the original.
When you delete the original and save and re-open you have a lumpy thumbnail in the design but on render, the original in all its clarity.
The lazy loading step is the killer addition.

Acorn