What the DPI of an image is
A digital photo is made of pixels. The DPI value is a note in the file that says how many of those pixels go into one inch when the photo is printed. A screen ignores it: a photo of 1200 × 1500 pixels looks the same at 72 DPI and at 300 DPI. Only the size of a print depends on it.
Change the DPI without losing quality
With “Only the DPI value”, the tool rewrites the number in the file and nothing else. The pixels stay exactly as they are, and a JPG is not saved again, so there is no loss of quality at all. What changes is the print size: 1200 pixels make 42.3 cm at 72 DPI and 10.2 cm at 300 DPI.
When the pixels should change too
Choose “Pixels too” when the photo must keep its print size at the new DPI, for example when a form asks for a picture of a given size at 300 DPI. The tool then enlarges or reduces the photo. Enlarging adds pixels but not detail: the print is as sharp as the original allows, no sharper.
Why print shops ask for 300 DPI
At 300 pixels per inch, a print held at reading distance looks sharp: the eye cannot pick out single pixels. What matters is how many pixels the photo has for the size of the print, not the number written in the file. A photo with too few pixels for a large print does not get better when its DPI value is changed.
72 DPI, 96 DPI and 300 DPI
72 and 96 DPI are old conventions from early screens. Many phones and programs still write 72 DPI into every photo, and a file with no DPI value is usually taken as 72 DPI. A photo at 72 DPI is not worse than the same photo at 300 DPI: it has the same pixels and only a larger print size.
DPI or PPI?
Strictly speaking, PPI (pixels per inch) is the value in an image file, and DPI (dots per inch) is how many dots of ink a printer puts down. In everyday use, “the DPI of an image” means its PPI, and that is the value this tool changes.