If you want to make sure that visitors to your site are made aware of new products or announcements, a popup display on the homepage or other popular pages can be an effective tactic. This benefit must be weighed against the fact that some people find popup displays quite annoying. Know your audience and only utilize popups if they truly improve the UX (user experience) or convey essential information.

To implement:
1) Install this free extension through Magento Connect.

2) Enable and configure the extension under System > Configuration > Featured Popup.

3) Add popup images under CMS > Featured Popup and set their dimensions, as well as associated store views. There are also several optional settings: opacity, priority (in the event of more than one popup being called from the same page), start and close delays, and an associated link.

4) After you have completed the above steps, you must update the XML for the page that you wish to display the popup on. Select that page from CMS > Pages and paste the below XML update:

<reference name="after_body_start">
<block type="featuredpopup/featuredpopup" name="toogasfeaturedpopup"  as="toogasfeaturedpopup"   template="toogas_featuredpopup/featuredpopup.phtml" />
</reference>

All done! Now a popup will appear on the page whenever someone visits.