I think it's more likely that a pixelated jpg is a result of the creator using the wrong format (ie, using jpg where gif/png-8 would have been more appropriate)

I think it's more likely that a pixelated jpg is a result of the creator using the wrong format (ie, using jpg where gif/png-8 would have been more appropriate) or over-compressing the image. PNG-24 is a lossless format, and generally shouldn't be used for production sites.