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.
Every Photoshop image encodes information in a rectangular grid of pixels. Some information, such as text and certain vector information (geometry), can be encoded differently, provided the image is saved in the PSD format.
Photoshop is an image-editing program – its images are composed of a grid of pixels and closely approximate continuous-tone photographs. Photoshop provides tools and commands used to edit images on a pixel-by-pixel basis.
In general, most operations in Photoshop are of the following form:
1. Use a tool or a command to select pixels within the image.
2. Perform an action on the selected pixels to change their color.