After creating an extensive animation with plenty of jpgs. pngs, and loads of effects I could render the project to swf with no problem. I then added a few still images at the start and end of the top level composition and rendering to swf bombed out immediately. On investigation as soon as it hit one of the new jpgs in the time line it quit - no error message given. After many wasted attempts at reformatting the jpgs and pngs I finally got it to work as follows.
I created a new layer with a black solid in it and made it the bottom layer. I gave it a fractal noise effect - and had to make sure it animated over the period of the animation using evolution jkey frames at start and end. I made the blending mode darken - which does nothing for a black solid. That allowed me to render the swf as long as I selected the raster option (which I didn't have to do before adding the jpgs!).
I'm not happy with this solution but will use it until I find a better one.