Even here at MR, some forum icons look pixelated.
Yep. That's because nobody has put in the time to render 2x versions of those icons. But if you scroll to the top and look at the MacRumors logo, it looks sharp as a tack on Retina, because they supplied a Retina version of the logo. It also looks like the forum "smilies" are Retina-enabled, too.
How about my personal pictures (especially baby/scanned pictures)?
Well, if you look at them in Safari, they are going to look pixel-doubled, because that is the way Safari was written to work on Retina machines. Unless the image is on a web page where it is told to render the image pixel-for-pixel on a Retina display (which is actually done by dividing both dimensions for the IMG tag by 2, on a device that has a web viewport "device pixel ratio" of 2 like the rMBP does), Safari assumes that images should be pixel-doubled so that they appear to the user to be the exact the same physical size on a 15" Retina display as they are on a 15" non-Retina display, because unless it is explicitly told that this is a Retina image, it has to assume that the image was originally intended to be displayed on a non-Retina display.
But if you look at your images in a Retina-aware image editing or display app that has the option of showing you the image at 1:1 (1 display pixel for every image pixel) instead of 4:1 (4 display pixels for every image pixel), then you can see your photographs without pixel-doubling. (Do note that when you do this on a Retina display, your photos will physically take up less space on the screen -- that is, they will appear smaller -- than if you viewed the exact same images on a non-Retina display. This is unavoidable...the pixels are physically different sizes on the two displays.) I don't know what apps those would be; perhaps somebody who is more in touch with the state of imaging apps on the Mac can chime in here. I would be shocked if Photoshop didn't have support for this by now, for example.
Interestingly, the built-in Preview.app appears to have some issues on Retina Macs, at least as of Mavericks. If you go to Preview -> Preferences -> Images, there is an option to "define 100% scale as: 1 image pixel equals 1 screen pixel", which also happens to be the default option. But if you open up an image in Preview and then go View -> Actual Size, it will pixel-double it! Preview did NOT used to do this on Mountain Lion. I actually have Mountain Lion on an external drive that I booted up on my rMBP just a little bit ago to make sure that my memory on this matter is correct, and it is: Mountain Lion Preview.app (and possibly Lion Preview.app as well) properly renders images at 1:1 on Retina/HiDPI Macs, and Mavericks Preview.app breaks this and pixel-doubles the same images.
I have Yosemite Public Beta 3 installed on a separate partition, but have not yet checked its Preview.app to see if they have fixed this on Yosemite. I will do that in a bit and report back on my results.
I think you can pretty easily get by with retina screenshots only. You can very easily convert them to loDPI (72dpi) versions with any decent picture editing program.
Well sure, unless you are concerned about absolute accuracy. For some uses, some people may not be, and your solution would be sufficient. But there are subtle differences between the HiDPI (2x) assets and the standard ones such that they won't look 100% exactly the same if scaled down (also, different image editing apps have different algorithms for scaling down, so you can't expect the same results depending on what you use), and even the versions of system fonts used on Retina displays are subtly different than their non-Retina counterparts; see
http://blog.justanotherfoundry.com/2013/10/lucida-grande-retina-optimized-in-os-x-mavericks/ for example. So if you want your screenshots for cMBP users to look *exactly* like what they should expect to see on their own system, then yeah, you will need to take specific non-HiDPI screenshots for that purpose.
-- Nathan
EDIT: Update -- no good: Yosemite's Preview.app is still broken in the same way Mavericks' one is.
