Low-res thumbnails with sips or Image Events

i work a lot with images, and often i use “Image events” or “sips” routines because its much easier than open some applications, however the negative side of using “sips” or “Image events” is that image files get low-res thumbnails.

example. look “Image events” or “sips” manipulated images in icon view, 252 px.
Some suggestions to add/rebuild icon layers, to improve the overall quality of thumbnails?