In order to know what resolution an image file will need to be – you will first need to know the size you would like to use the image at (for example 4" x 6").
As our recommended pixels-per-inch (PPI) is 300, multiply these dimensions by 300 to find out the minimum recommended resolution (in pixels) for that image.
Hence, a 4 x 6 inch image should be 1200 x 1800 pixels (or larger), as calculated below:
Width: 4" x 300 = 1200 pixels
Height: 6" x 300 = 1800 pixels
See: Image quality