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#11
The DPI for screen reading is 72DPI for Apple MAC and 96DPI for PC 300DPI is used for print purposes
when you create an image for the web you should save in either JPG or PNG format and set the compression ratio to whatever quality you find acceptable PNG is a later format than JPG and therefore gives better results generally, although if quality is not so important JPG can be made smaller.
Basic rule is the smaller the file - the lower quality the image.
One technique used on the web is to have a small low quality image that works as a link to an second higher quality larger image which acts as a pop-up, (this is what is seen in the magnified images used on Amazon and other similar sites using PHP, Java Etc)