The "image resolution" correspond to the number of pixels contained per inch (1 inch = 2,54 cm) and is expressed in DPI (= dot per inch). 
The higher the number of pixels, the higher the quantity of information or precision in the image. For instance a 300 DPI image corresponds to 300 (length) x 300 (height) pixels image, which means 90 000 pixels. 
This data will help figure out the maximum size of a print. 

The image size to be printed is usually not over 300 DPI, and the further the distance of reading the print, the lower can be the resolution of the image - which is linked to the resolution of the human eye.

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