As les28 says they are probably small (low resolution) photo's.
Try enlarging by a very small amount then use Picasa to export to a folder. (use export not save as that way you create a new photo)
Now take that exported picture and do the same again, using a different name (e.g. pic 1 , pic 2, pic 3 etc)
By this method you will gradually increase the picture quality. It may take some time and there will be a trade off between quality and size, but they should be better than a straight 'Enlarge at once' picture.