OK. I'm sure that this has been asked a million times before but it's not going to stop me asking it again!!!! 
If I create a header image for Facebook, on my standard iMac's screen, I'm producing it at 851px x 315px which is the required size for images created at 72dpi. If a Retina screen shows exactly double that of a standard screen (144dpi), I see one of two things happening - either the pixels of my original 72dpi image will be blown up by a factor of two, which means that the perceptual resolution will be halved to 36dpi, or my original 72dpi image will show on screen at half it's original size (ie tiny) but at twice the resolution at 144dpi. So my query is - does an image at 851x315 designed to be seen on a standard display ALSO need to be accompanied by one saved at 1702x630 (twice the physical size) which would suit a Retina version? In the case of the latter, would Facebook display it properly?
If I create a header image for Facebook, on my standard iMac's screen, I'm producing it at 851px x 315px which is the required size for images created at 72dpi. If a Retina screen shows exactly double that of a standard screen (144dpi), I see one of two things happening - either the pixels of my original 72dpi image will be blown up by a factor of two, which means that the perceptual resolution will be halved to 36dpi, or my original 72dpi image will show on screen at half it's original size (ie tiny) but at twice the resolution at 144dpi. So my query is - does an image at 851x315 designed to be seen on a standard display ALSO need to be accompanied by one saved at 1702x630 (twice the physical size) which would suit a Retina version? In the case of the latter, would Facebook display it properly?
