Gary: I don't think that it is a good idea (or should be necessary) to optimize the images before export, as this is a lossy
operation which limits design freedom – if one in the course of the design process decides to increase the size of images
again one will end up with nasty pixelation.
Yes, that is a good point. I always keep copies of the original images for this reason. And you can always drag and drop the original image onto the optimized image to restore the original resolution.