fyi,
the PNG-spec also supports compression but unfortunately not many user agents (ie. browsers, etc) do. Also PNG-format allows to keep editing information intact (vector graphics, layers, etc). If a PNG-file would only contain vector graphics, even uncompressed it would be considerably smaller than a JPG.
Now that we've got that out of the way, for screenshots the only format to use imo is indeed JPG, simply because it's easy, widely supported and it's compression is perfect for the job.
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