If you're talking about flash, whatever you place within the object tag will show up if flash is not supported.
Here's an example of a flash movie with a fallback image:
<object type="application/x-shockwave-flash data="yourmovie.swf" width="400" height="300">
<param name="movie" value="yourmovie.swf" />
<img src="noflash.gif" width="200" height="100" alt="" />
</object>
I typically test for plugins using javascript and write to the page based on the test results.