Open your Google Drive and upload your image in big format
Screenshot of image folder
Share the image to anyone with the link and copy the link
Screenshot of to context menu to share the image
Open the link in your browser and click on the 3-dots-menu to get and copy the embed link
Go to your Google Site and add a new page as full page embed
Screenshot of the context menu to add a new page (full page embedding)
Choose the left option
Screenshot to add content to the full page embed
Paste in the copied link (not iframe code entirely ) to your image
Screenshot to add content by URL
It will look like this
Screenshot of embedded content (big image)
Then go to the page, where you like to have the smaller version and use the insert image option and select it from you Google Drive
Screenshot of disc menu to add an image in a Google Sites page - Double click on a blank space to open the menu
After adding it to your page, add a link to it
Screenshot of the image settings (context menu) - Double click on image to open it
Select the newly created page
Screenshot of selecting the created big image page (internal link)
Open the preview of your Google Site to check the new addition
Screenshot of the preview in the Google Sites editor - Control if everything is working
You can click the image and the full page embed will open in the same tab by default, but the user can use the go back button in his browser, or right click to open the link in a new tab by himself
Screenshot of the preview of the full page embed
As you see, the page shows your websites navigation at the top as further alternative to browse your website and the image is sized to be shown completely, but you have to option to zoom in/out as you like.
Screenshot of the test of the Google Drive viewer zooming capabilities
With the link button in the upper right corner of the image, you can open it in Google Drive viewer directly.
Screenshot (comparison) of the image opened in the Google Drive viewer directly
There you also have the option to print and download the image in original quality, if you have allowed it in the sharing settings.
Screenshot of file settings for the image in Google Drive - (Dis-)allow printing or downloading