How to add link to large image

I generally use images that are 640 x 480 throughout my blog, so they load quickly and don’t take up much space on the host’s server.

I would like to add a link to open a large version of the chart on this page, Life in a Dol house blog, to make it easily readable. The 640×480 is too low resolution.

I can’t find a way though to create a link on the page to the larger image, which will open as an overlay on the existing page.

Any help to do this would be much appreciated. Thanks for reading.

The page I need help with: [log in to see the link]