I think you're out of luck. If your pic isn't hi-res enough you're never going to get good quality out of a blow-up. You only have so much information to work with in those pixels. By blowing up your photo you're just making individual pixels stretch to two or three pixels.
You could do something with the "blur" tool on Photoshop, but in the end you're just going to make your pictures look kind of "plastic" by doing that - probably not what you want.
Have you considered asking this question in the Logos forum? There are many graphics experts there who could answer this question better than we could. It's worth a shot.