Every Photoshop image is rectangular in shape, corresponding to a fixed measurable size (in inches), and exists at a fixed resolution (usually expressed in pixels per inch).
72 pixels per inch is appropriate for on-screen display (e. g., websites).
300 pixels per inch is appropriate for printed photographs.
600 pixels per inch is appropriate for printed vector drawings and/or text.
Image resolutions greater than 600 pixels per inch might arise in the case of scanned slides.
Use Photoshop’s Image > Image Size command to change an image’s measured size and resolution.