Digital images do not have, or need, a "Resolution" value. Digital images have Pixel Dimensions.
Only when you print your photo to paper (or Re-size an image) might Resolution be considered for "quality" reasons. (eg. Does your image have enough pixels to render a quality image at the paper size you are printing?)
I like to think of ppi and dpi as totally separate things-
"ppi"- The number of image pixels that will occupy 1 inch or print paper. (eg. 300ppi)
"dpi" - The number of INK squirts (dots) that the inkjet printer will place within 1 inch of the paper. (eg. Printer capable of 1400dpi)
The article in this link should be headed The Myth of PPI !! the author is writing as if ppi and dpi are the same thing, but it does help to explain 'ppi'.