PHOTO RESOLUTION
This is one of the trickiest areas and the source of most problems when trying to get photos on the web. Scanning photos is one of the most popular ways of obtaining photos, but scanners have been designed to capture very high resolution images, typically for printing on high resolution printers. High resolution means a large file size, and so most typical files from scanners are unsuitable for use on web pages. Modern digital cameras produce equally large files, typically many thousands of pixels across which are also too large for websites.
Photo resolution is measured in dpi (dots per inch). The rule of thumb to remember when scanning photos is to scan at around 72dpi to 100 dpi as that's the approximate resolution of your computer screen. So if you have a 4" by 6" photo and you want it to come out about the same size on your web page (which is probably too big) then scanning at 72dpi will be about right. Typically modern scanners can scan at 300dpi or more and produce huge files. Increasing the resolution can make a huge difference to file sizes. 300 dpi is more than 4 times the 72 dpi resolution, but because a photo is square these result in files than are typically 16 times the size! The only reason for scanning at resolutions higher than 72 dpi is when you want to enlarge a photo or a part of a photo. Scanning at over 200 dpi is almost always too high a resolution. If you don't want to bother about resolution and find it all too confusing, then stick with this simple rule of thumb; If you're producing a JPG file that's more than, say 100k, that's almost certainly too big for the web. Also, if your image is more than 1000 pixels across it’s almost certainly too large for use on the web.
WHEN TO SCAN AT HIGHER RESOLUTIONS
If you want to show just a part of a photo, or you plan to use photo thumbnails that open a separate window onto a higher resolution version, then it can make sense to have larger files. Also sometimes it's better to have a larger, higher resolution photo and reduce its size before you use it on the web - this can produce better quality images. Golden Rule – scan printed photos at 72 to 100 dpi. Higher resolutions if you want to enlarge just a portion of a photo.
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