It was literally five SimplePie questions ago. Did you even search? If you searched you would have found an answer very similar with actual working code, showing how to look inside the content and find the first image or any image...
I've answered this question a half dozen times, someone do something to amalgamate them, someone with StackOverlow-fu.
The short answer is you have to parse the HTML. I'm not sure you don't mean the image is in the content not the description, regardless, you use some something like html_simple_dom to step through the HTML and find the image your want.
If the feed supported enclosures or thumbnails or was part of the MediaRSS standard this extra effort might not be necessary, but not all feeds are created equal. I know I've given an even better answer somewhere just search this site or Google more.