Pixels
If you enlarge this picture, you can see alot of pixels and dots all over the picture.
You should render your fractals higher. From what i can see you render them around 100 (could me more/less)?
Rendering in apophysis or any other program should be around 1000 or more and then you get an good quality of your fractal.
Try this and re-upload the picture, you'll see alot of difference in:
-Nearly to no pixels or dots anymore.
-If you enlarge the picture the quality should be the same.
-Looks get better overall.
Grtz