First of all, resize/resample, print size/pixel size, and the like is the
basis of digital image manipulation. Perhaps study up in archives here
and at scantips.com, where the subject usually is made clearer for
However, 4256 x 2848 is 12 MegaPixels, so I don't know how you are
getting that from your 6 MP camera? Does it have an additional "high rez"
option whereby IT is upsampling/interpolating?
Which answers your question: "where do the extra pixels come from?" They
are interpolated (made up, created) by sampling from surrounding area.
Generally of limited value, often not good at all.
The simplest trick is to use your existing original pixels only. When you
change to 300ppi, uncheck resample, and it will give you the largest
print area at 300ppi that you can make, using original pixels only.