Add HTML Snippet to existing image

As for as linking to the page within iWeb it will depend on how you publish your site.

if the pagename of your popup window was "popup" you would select the text or image you want to use as the link then in the link inspector check enable as a hyperlink, Link to: External Page, then the published page address. Example:

http://MyWebSite.com/popup.html or if using iWeb builtin webpublishing it would be some thing like this: http://MyWebSite.com/SiteName/popup.html

Removing page navigation from the popup will require you adding code to your page or pages & results may vary info on popup scripts: http://www.htmlcodetutorial.com/linking/linking_famsupp_70.html

*edited

Here is another example of mouse over text that could be done with a html Snippet

hold you mose over "Who am I?"

http://jeffnitschke.com/mouseovertext/About.html

A popup in a new browser window has to have some controls so that the visitor can close it.

Using a lightbox or a modal will give you a popup which doesn't require a new window.

Here's an example of some modals in iWeb...

http://www.iwebformusicians.com/iweb-snippets/modal.html

If you want to use a tooltip you can style and position them with CSS...

http://www.iwebformusicians.com/iweb-snippets/tooltip.html

The problem is that they are not too good on mobile devices!

If you are using a html snippet, then you need to create the link using html.