Neophyte said:
If I have a picture that is 150 dpi, can I change it to 300 dpi? If
so, how?
If you find a 300dpi version, sure.
Seriously, 150dpi means that there are 150dpi of detail.
You cannot magically add detail.
Consider: You had $300 (a 300dpi image). To make space, you threw away
$150, taking you down to $150 (dpi). There is no magic way to get the $150
back to get $300 again.
Another analogy: Images are like arrays of grains of sand, all different,
arranged into a large grid.
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
ABCDEFGHIJ
That is a 10x10 pixel image. At 150 dpi, that is 1/15 inch.
You could increase the dpi to 300 by reducing the print size to 1/30 inch.
The only way to have a 300dpi image at 1/15 inch would be to add extra
pixels
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No-one knows what to add where the question marks are.