The Thumbnailer add-on for BlueGriffon lets you insert a link to an image showing only a thumbnail for that image. You can easily control the maximum width or the maximum height - or both - of that thumbnail. The thumbnail can be output as a new PNG file onto your hard disk or as a
data:// URL directly embedded into the HTML document.
For those of you using toolkits like Lightbox or jQuery plugins to show images in a layer appearing above the document, it's also possible to directly assign a class to the link to make the toolkit recognize it automatically.