'Image Size' - the most confusing dialog box in Photoshop
First, a question... according to the 'Image Size' dialog, what's the resolution of the image on the right?
An understandable, but incorrect answer would be "72 pixels/inch". The right answer is "20MB", or 3064x2282 pixels, which at 8 bits/channel, works out at 20MB. It comes to the same thing.
With digital, just like film, image resolution is the measure of how much information is contained in an image. That means we're talking Megabytes - it has absolutely nothing to do with pixels/inch.
So, what's going on here? It quite clearly states 'Resolution 72 pixels/inch'. But notice that the 'Image Size' dialog is really two boxes in one. At the top it gives us Pixel Dimensions, or image resolution in Megabytes. At the bottom, it gives us 'Document Size', or print resolution in pixels/inch. The two are only related when we come to print the image.
The 'Image Size' box is telling us that if we were to print this 20MB image file at 72ppi (above) the printed image's dimensions would be 108.09cm x 80.5cm. If we were to print it at 300ppi (below), then it would be only 25.94cm x 19.32 cm. Note that the 'Resample Image' checkbox is off and also note that the image resolution is unchanged at 20MB. Only the document size has changed.
So, the rule is: when discussing image resolution, we always talk about Megabytes. When talking about print resolution (or any other type of output), we can refer to pixels/inch.