The short version
On screens, only pixel dimensions exist. A 1200 × 675 px image is displayed from its 810,000 pixels — whether its metadata says 72, 96 or 300 DPI. Browsers never read the value. Anyone tweaking DPI to fix a website image is turning a knob that isn't connected to anything.
Where the myth comes from
The original 1984 Apple Macintosh monitor really did show 72 pixels per inch, so "72 DPI = screen resolution" was briefly true — in the eighties. The rule survived in design curricula and "export for web" dialogs long after monitors moved on (a modern phone shows 400+ pixels per inch). It persists because it sounds technical and is easy to remember.
What DPI actually does
DPI only matters when pixels have to become physical dots on paper. It's a conversion factor: print width = pixel width ÷ DPI. A 3000 px image at 300 DPI prints 10 inches wide; the same file at 72 DPI prints 41 inches wide (and each dot is correspondingly coarser). The image data is identical — DPI just tells the printer how big to make it.
What to check instead
- Pixel dimensions — does the image have at least as many pixels as the space it's displayed in (×2 for Retina-crisp logos)?
- File size in KB — is it small enough to load fast (content images under ~150 KB, heroes under ~200–500 KB)?
- Format — WebP for website photos, PNG only for transparency, JPG for email.
Those three numbers determine how your image looks and loads. DPI determines nothing — on screen it is, literally, just a note in the file that nobody reads.