Rendering the same scene at the small size (200 pixels for the longest edge) gives a pretty nice thumbnail. Then convert this .bmp into a .jpg compressed to about 62%. Some of my best ones are done this way - and load quick. Another way is to render at a maximum of 800x800 pixels and then reduce your picture to 25% of it's original size and save to .jpg as above. 50%. 25%, 12.5%, etc. nice and clean. Noel