Notes on Image Quality
Retina displays have twice the pixel density of traditional displays, allowing for finer details to be shown. This in turn requires larger desktop pictures.
Some common (2017) retina display sizes are:
- MacBook: 2304×1440 pixels (resolution = 1152×720)
- MacBook Pro 13": 2560×1600 pixels (resolution = 1280×800)
- MacBook Pro 15": 2880×1800 pixels (resolution = 1440×900)
- 4K display: 3840×2160 pixels (resolution = 1920×1080)
- 5K iMac: 5120×2880 pixels (resolution = 2560×1440)
System Preferences and macOS will normally report the (lower) resolution size as the display size. This may seem somewhat odd, but reflects the physical amount of space available for the UI (User Interface) which is the same for both retina and non-retina displays.
Generally Avoid upscaling images. A desktop image should ideally, be at least the same size (in pixels) as the display (or displays) it is intended to cover.
Small upscales (10–30%) will often look ok.
Some images you'll find will likely already have been upscaled making them bad candidates for future upscaling. Such images will typically lack crispness and instead look blurred or smudged.
These are sometimes the result of sites trying to be helpful by supplying the image in multiple resolutions, even when this means excessive upscaling. They can also be found on forums and on image boards, where people have tried to make small images work on much bigger displays.
Some image formats like jpg are lossy, meaning that higher compression rates leads to smaller files but also to more visual artifacts as some of the original image data is discarded/lost.
Be wary of overly compressed images, such images will often look grainy or pixilated due to their compression artifacts. Upscaling them will typically make this even more apparent. Some image hosting services (e.g. imgur.com) re-encode uploaded images aggressively, to save on storage and bandwidth costs.
Also note that lossy images get worse if they are re-encode multiple times, this may happen when:
- Person A shares some images via a site like imgur.
- Person B downloads these.
- Person B re-shares some of these via imgur to person C who does the same thing and so on and so on …
For the best image quality, download directly from the artist or official site. Use a reverse image search to track down unknown artists and sources.