The dpi or ppi is not a relevant quantity -- setting it in the image file is completely meaningless and doesn't change the image size or resolution. The dpi/ppi setting in a file is just metadata (like the author, or keywords, or camera model which is stored in image files). What matters is total pixels. If I have a 1500x1500 pixel image and I set the dpi to 300 or 72 or 5 million, it doesn't change the resolution of the image which is 1500x1500 pixels.
A printer might ask for a 300 dpi image and what he means is, he needs enough pixels so that when he prints he'll have at least 300 dpi, eg. if you give him a 1500x1500 pixel image, then he'll be able make a print 5"x5" at 300 dpi.