From: My Own Private Hell
The motion effects for stars and other stellar phenomena are stitched together from a series of .png files, 300 for star coronas, 100 for black hole, etc. These would need to be recreated at a higher resolution in order to remove the blocky effect, but I don't have or know of a free program where that is possible. I don't have Photoshop unfortunately.
Do you mean starting from scratch with the image in question, or do you mean increasing the res for image? (eg, from 1000x1000 to 5000x5000) If so enlarging, increasing the res, is not the way to go, that will only make it worse unfortunately. I use Irfanview for image manipulation, it can stitch images, resize etc and to be honest its better at most things in that respect, once ive got the image and sized, cut it etc I drop it into PSP9 for final adjustments.