1. Whatever is saving the pictures is not setting the DPI to a nominal
value, or the correct value, whatever that might be.
I think that what you said is that your scanner application program sets the
DPI to some reasonable value.
DPI is a Printing/Publishing value, and is usually above 100 and less than
400 for most uses.
It should have nothing to do with how a picture is displayed on a pixel
based display.
It also has minimal use when printing to a consumer inkjet/bubblejet
printer. Such devices are "GDI" and RGB based, and the pixel (PPI)
value is more important. This is a result of specifying the physical size on
paper of the picture during a layout process.
Many of the printers have options to expand or shrink a picture to a full
page, or a specific size on a page.
Most of the applications used to edit a photograph also have the ability to
set the printed size of the photo.
Some of the applications can also reduce the resolution (size in pixels) of
the picture, usually done to make the picture suitable for web display or
attachment to an email, etc..
Having said that, some applications were written without a correct
understanding of what DPI is really intended to be used for.
Most decent applications that are intended to be used to print a photograph
are not dependent upon a DPI value imbedded in a digital photo file.
Some digital cameras force the DPI value to 72 or 96. This is an artificial
value, and has nothing to do with the true pixel resolution of the picture.
(If a digital camera sensor were one inch across, and captured a picture
that had 3,200 pixels across, it's obvious that a DPI of 72 is silly.
Remember that an 800x600 pixel picture can be printed at a resolution that
fills a specific size on a page or the whole page.
Also, a printer may print with a "DPI" (actually pixel) equivalent that is
quite different than you would expect, due to processing in the printer
driver.
You did not mention what application you are using to edit/crop a picture,
so any advice on how to set it up cannot be given.
