Resolution

Images are measured in physical size and resolution.
For most uses an image will need to be a specific size.
The true size of an image is measured in pixels.
For printing resolution is important. Pictures can be re-sized and their resolution altered for these purposes.

Original image from Canon 10D (6.3 megapixel)

 

Same image showing the size in inches at 72 dpi.

 

Same image showing the size in inches at 300 dpi.

 

Resized to 4500x3000 pixels


 
 

Resized to 4500x3000 pixels,
showing the size in inches at 300 dpi.

  For printing an image is almost always saved at
the print size eg 10" x 8" at 300 dpi (ppi)
 
For screen use working in pixels is more accurate as sizing in inches or millimeters will not work as screen resolutions and dpi will vary.