Image resolution is often discussed as if it were one number, but several related ideas get mixed together: pixel dimensions, printed size and pixel density metadata.
Pixel dimensions
An image that is 2400 × 1600 pixels contains a fixed grid of pixels. For web use, those dimensions are usually the most direct description of how much raster detail is available.
What DPI or PPI means
Pixels per inch describes a relationship between pixels and a physical display or print size. A metadata value such as 72 or 300 does not magically add or remove pixels from the file. The same 2400 × 1600 image can be assigned different print-density metadata while retaining the same pixel grid.
Resizing down
Reducing pixel dimensions can make a file much smaller because fewer pixels need to be stored. This is often one of the highest-impact web optimizations when a camera original is far larger than the display area.
Resizing up
Increasing dimensions requires creating new pixels from existing information. Modern upscaling can produce visually pleasing results, but it does not create the same genuine captured detail as a higher-resolution source.
Practical rule for websites
Start with the maximum display size you actually need, account for high-density screens where appropriate, and export a file that is large enough for that use without sending thousands of unnecessary pixels.