You can insert text or an image using :before and :after.
To insert text before something:
h1:before{content:"Nice";}
This will insert the text into all h1 elements on the page. Of course, this same method can be used with :after.
Similarly, you can insert an image as well:
.alerts_popup:before{content:url(
/images/upload_gallery/10841.jpg);}
Please note that if you use an image, there's no real way of scaling it. There is a StackOverflow question on this topic, but whether it's compatible with the DSi is another matter.
