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Thumbnails at a high pixel density. I just want them up fast. Any quality that can be squeezed out of it is a bonus.

JPEG has a fixed macroblock size (16x16 pixels), which negatively affects high resolution low bitrate images.

If you must use JPEG, I suspect you might get better visual quality by halving the resolution and upsampling on the client.

By doing so, you are effectively setting the lower and right halves of the DCT to zero (losing all high resolution info), but get to have 32x32 pixel macroblocks which lets you better make use of low frequency spacial patterns.

Oh, that's interesting. I typically serve thumbnails at 2x resolution and heavily compressed. Should I try to instead compress them less but serve at 0.5x resolution?

I'd say it's worth a try.

I recently noticed that all the thumbnails on my computer are PNG, which I thought was funny.

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