This can be done as follows:

1) Add an image to your page - this image will be replaced by the embedded web page when you view the site in your web browser.
2) Select the image then go to the "Tools" menu and select "Custom Script Assistant".
3) Select the "Object" events button then enter the following code in the "Contents" box:

<iframe src="http://hostname/xxx.yyy" width="200" height="200"></iframe>

...but replace "http://hostname/xxx.yyy" with the URL of the page you want to display. You will need to set the width and height appropriately for your design.

4) Click OK.

When you preview the site, the page you specified should appear in place of the image.