As mentioned PPI is print resolution and has no effect on photo size for electronic display.
PPI and image resolution (xxxx px by xxxx px) determine print size.
1500 px / 100 pixels per inch = 15 inches.
1200 px / 100 ppi = 12 inches.
Many people are satisfied with the quality of prints made at 100 ppi, and 100 ppi is around the minimum print resolution most online print labs are willing to print at.
As print resolution increases, print size decreases.
1500 px / 200 ppi = 7.5 inches.
1200 px / 200 ppi = 6 inches.
The quality setting affects the file size, but not the image resolution.
Anyone copying your photo off the Internet can change the print resolution value.
If you reduce the image resolution before putting a photo online anyone copying your photo off the Internet can increase the image resolution. There are some software applications specifically for that, but increasing image resolution can be done with most image editing applications. There are limits to how much image resolution can be increased using software.