If the Flash is inside <object> tags, you can have a fallback child element inside <object>. Something like this is common:
Something like that is how the object element is supposed to work. You're using an iframe but I'm still assuming that inside the iframe is still an <object> tag (or possibly an <embed> tag, which I dunno if those can have fallback children).
<object id="someFlashThing" type="application/x-shockwave-flash" width="800" height="200">
<param name="movie" value="theFile.swf">
<p><a href="optional link to more portfolio images, if they exist"><img src="anImage.png" width="800" height="200" alt="Either what the image is showing, or if a link, where the link goes"></a></p>
So, ideally this is something the browsers should do automagically for you, so long as you simply provide the image. This way, not just iThings but any other browser who happens to not have the Flash plugin will get an image.
And then, as in my example, if there's something non-Flash you can link to that would be nice for folks.