Porque nunca deberías grabar un JPG con valor 7, en Photoshop

Why You Should Never Save a JPEG as a 7 in Photoshop

We've been told for years that file size doesn't necessarily mean higher quality, though it's usually regarding megapixel counts on camera sensors. But did you know thatthere's a point in Photoshop where increasing the quality setting—and file size—actually decreases the image quality? That point is 7.

When saving JPEG files, Photoshop useschroma subsampling—which hacks off color data because your brain doesn't register it at the same rate as brightness—up to 6. But after that it saves all color information. To keep the file size progression linear, rather than having a humongous size bump moving from 6 to 7, files are more heavily compressed at the 7 setting than they are at 6. The quality scale should right itself if you move to 8, though; it's just the abrupt change in the amount of color data at the 6/7 gap that causes the muck up. See? Less really is more, except when it's less, or when more is also more. [ImpulseAdventure viaPetaPixel]

Fuente: http://gizmodo.com/5835415/why-you-should-never-save-a-jpeg-as-a-7-in-photoshop