Right. Completely ignore the dpi/ppi setting for photos until you are ready to output them. What is important is the resolution or number of pixels total (3000x2000 etc).
After you've saved your original and edited files somewhere safe and are ready to either print your image or resize it for putting into a printed document, then dpi comes into play. Say you have the aforementioned 3000 x 2000 image. If you choose to print an 8 x 12, you would go into, well lets say photoshop, choose resize, and without resampling the image, enter the dimensions you wish to print, which would give you a 250 dpi image. Or say you were putting the image into a word processing document, you could specify the size as 4 x 6, which would give you 500 dpi. Since most printers can't resolve that high a dpi level, you're wasting file size, so you would then resize in Photoshop with resample on and set your dpi to match the settings for of the printer.
So depending on how you set it in photoshop, without resampling, that 3k x 2k image could be:
40" x 60" @ 50 dpi
20" x 30" @ 100 dpi
10" x 15" @ 200 dpi
6.67" x 10" @ 300 dpi
Don't forget screen pixels do not equal printer dots... a popular inkjet printer might advertise 2400 dpi, but you can't print 2400 individual pixels per inch - the printer instead takes your 100/300/xxx dpi image and then internally creates a dot pattern of whatever colors the printer uses, much like the halftone color pattern in the newspaper. So it might take a matrix of 8 printer dots to reproduce 1 pixel as displayed on the screen.
In actuality screen display should be listed as ppi (pixels per inch), but unfortunately Photoshop and other applications have used dpi (dots per inch) as a catchall for so many years that dpi is often used interchangeably for ppi which can really confuse the issue for some people.
It can be confusing I know, but just remember as far as the image goes, its the number of pixels that is important, not the dpi/ppi that is reported by your application.
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