Measuring minimum resolution in Photoshop/GIMP

Question

I am following Apple's iTunes connect guide (as of today 13th November 2013) for designing an image. At page 239 of their guide I am confused on the following:

Background artwork must be at least 3200 pixels in width and 600 pixels in height with a minimum resolution of 72 dpi, in the RGB color space, and in the PSD format.

How can I make sure (measure?) that all the layers of the PSD file have a minimum resolution of 72 dpi?

In other terms, how can I verify that the file does not contain layers that had been created by scaling lower resolution images and then setting the overall file resolution to 72 dpi ?

Answer 1

Photoshop layers are all controlled by a single ppi setting.

When creating a new document, you are asked what PPI (Resolution) you want the document to be set at:

If you are working on an existing document, the PPI setting can be found at Image > Image Size... in the menu:

Note that if you change the PPI (Resolution) setting with Resample Image unchecked you will see a change in the width and height of the image as well.

Answer 2

The image size does not depend on pixel dimensions but it determined by the dpi value the images will have on the output device. For a computer monitor this will typically be 72 dpi which is the default setting in most graphic applications. For print output other dpi values may be needed.

These settings can be made or looked up in Gimp with the Image > Print Size... dialog:

Note that depending on image size there may be rounding errors in the value displayed. Also note that after we changed the dpi setting in Gimp all newly created images will also have these settings. We therefore will have to change them back to defaults after we had saved our image.