a process whereby file size is optimised for the web (usually reduced).

You can freely copy and use works in the public domain. Images are in the public domain if copyright has expired or they have never been protected by copyright.

Under New Zealand law copyright expires 50 years after the death of the author, artist or photographer. If the image belongs to the crown then copyright expires 100 years from the end of the year in which the work is made. However Cabinet has directed that all crown works should be made available under a creative commons licence. See NZGOAL at: http://nzgoal.info/

If the image you want to use is a photograph then you may use the DigitalNZ copyright status flow chart to determine whether or not the photograph you want to use is out of copyright. Photographs taken prior to 1944 are considered out of copyright. See:

http://makeit.digitalnz.org/guidelines/enabling-use-reuse/copyright-status-flowcharts/

Some government works are in the public domain for example, the United States Government offers a number of images covering a wide range of topics, from medicine to agriculture, which are not protected by copyright and which free to copy and use: http://www.usa.gov/Topics/Graphics.shtml

Signified by PPI (pixels per inch) for screen resolution  or DPI (dots per inch) for print resolution.

Note: when talking about monitor resolution, people sometimes refer to a measure of pixel height and width.