Image size and resolution vocabulary:

DPI, pixel dimensions, document size, resample, color space, bit depth, canvas size.

Know these terms and how they relate!


Example of how DPI doesn't really matter on the web. Take a cat image with these dimensions

I'm going to set the pixel dimensions to 800 x 533, notice how the size went form 40M to 1.22M.
Left the resolution set to 72, the default on my camera.
Image_Size_ first edit.jpg

Here is the image
cats 800 x 532 72 dpi.jpg

Unchecked the resample button (So the file didn't become too large for the wiki) and changed the DPI to 300, note how the width and height of the document size changed.

second edit.jpg

Here is the new image, exactly the same as above, in pixels and file size
cats 800 x 532 300 dpi.jpg

The computer and its display setting determine how DPI is displayed, so you setting the resolution in PS won't affect that (though this may change in the future........)

Some common iphone resolutions

iphone resolutions.jpg

This page presents a good comparison too on JPEG quality.