Insert the image. The image I used is 800 px wide.
You will see the "Link: Go to link..". and "Image: Align..." lines. If you don't, click on the image.
Select S, M or L for the size. ( I used S).
Click "Go to Link"
Copy the URL of the full size image.
Save the page.
Create a new page for the full-size image.
Insert the image using the URL you copied.
On the Link: line on that image, click "Remove" so it is not a link to itself.
Save that page.
Edit the page with the small image.
On the Link: line click "Change" and enter the URL of the full size image page.
You can link to ANY page instead of a page containing the large image. Example: the second image on this page.
I edited the HTML to add alt and title properties to the image so when you hover over it you will see the text.
I made the large image page a sub-page of this page so that there is a link back to this page automatically. Or you could add your own link there.