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#301
With creation of images for screen, (Web) display the actual resolution of the image is largely irrelevant ...
An image for use as, for example, a HD wallpaper is set at 1920 x 1080 pixels - the image quality for curves Etc, that this image is displayed is more to do with the size of the screen used to display it. An image will always look sharper on a small screen than a large screen, given that the resolution is equal. As the screen size increases the pixel size increases so at the same screen distance the pixels will be more visible.
The "rules" for internet images was always set at 72DPI as this was the dot resolution of early MAC screens whereas the PC used 96DPI, this system is these days of less use and it is "better" to design for the native pixel resolution of the screen that is the target output, looking at the size of objects in an image as a percentage of the screen, rather than an actual size.
High resolution images are still required for print output where the output device is capable of much higher dot pitch - for print it is usual to set your "canvas" based on the size of the output media times the resolution. Thus a 7"x 5" print on a 300 DPI printer would need a page size of 2100 x 1500 pixels, to achieve the best quality output.
If the pixel size of an image is less than the native size of the output device then the result will be less than ideal, and will look jagged as it has to be resized upwards to be displayed or printed, resulting in the pixels being interpolated (guessed), to fit - also there is no benefit in creating images too large for the output device as the image will need to be resized to fit and this will leave the final quality to the software in the output device, used to resize the image
When I take a photograph the image is 6000 x 4000 pixels so to best display on a HD TV screen I have to consider both the screen ratio and the resolution. if I print the final image I adjust the resolution to suit the output device, (on a 300 DPI device the ideal paper size would be 20" x 13.33"), and the size required. This can be left to the printer driver but is often better performed within the graphics application output module or manually by cropping the image to give the correct pixel resolution