A common misconception about DPI: there's nothing about digital bitmap file formats that directly define the print resolution or quality. To raise the resolution of images, you must (1) allocate a larger number of pixels, and (2) tell the applications to apply the pixels to a specific physical print/display area of your choosing.

For example, a 64x64 pixel icon printed at 1"x1" will obviously look poor compared to a 1200x1200 pixel photographic image printed at the same 1"x1" final area.

Some image formats may offer hints to the applications about intended print area (aka DPI fields). Some applications even allow you to adjust those fields with some GUI controls. It's still up to the applications (and device drivers) to ignore or honor these hints and really apply logical dots to physical dimensions.

All of this is typically moot when discussing images on the screen. The user's screen will have a certain resolution of their choice (e.g., 1600x1200 pixels), and a certain viewable area defined by their hardware budget (e.g., 19" diagonal). If the viewing application doesn't scale the images accordingly (and web pages/browsers generally don't), then a 300x300 pixel image will be huge on a cheap display and tiny on an expensive display.

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