JPEG is better for photos because although it is lossy, the compression is much better and it is designed to take advantage of the way the human brian sees an image so that the data discarded is not so noticable. Certainly with large files I'd use JPEG. The difference even between a 100% quality JPEG and a 75% quality is not every noticable in most conditions but can represent as much as 60% compression.

So I say PNG for graphics and JPEG for "natural" or large images.

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mysql_query("DELETE stupidity FROM earth") or die("Stupidity");