I'm confused about the meaning of term PPI (Pixel Per Inch). It should mean two different things:
- the resolution of a monitor screen
- the ratio of the number of horizontal (vertical) pixels of an image and the size in inches of the image width (height)
If the above is correct how the PPI of an image is related to the PPI of a monitor screen ?
Those 2 definitions aren't mutually exclusive. An image and the monitor it's displayed on can have different resolutions, but the monitor's resolution will be the upper bound (the image will be displayed at either it's resolution or the resolution of the monitor - whichever is lower).
That is unless any scaling is used. For an overview of scaling techniques see: Wikipedia - Image Scaling.
The image's PPI will define the resolution of the image independent of the monitor it is displayed on. The actual resolution you will see when viewing the image on a monitor will depend on the monitor's resolution, the image's resolution and if any scaling techniques have been used