Help!!! How can I auto add images to rss feeds (posts) that don't have images?

I have several rss feeds into my site and want to automatically attach a featured picture to each post from each feed. ie I want to attach picture 1 to all feeds from feed 1 and picture 2 to all feeds from feed 2. So, the same picture will be used on all feeds from the same source.

Is there a plugin out there that will enable me to do this?

Any advice would be very much apreciated.

Thanks.