Because digital images have no physical dimensions, the dots per inch are merely ballpark figures used as a starting point when scanning. 75-150dpi is great for images on the web. Scan at 150dpi and reduce slightly to the dimension you need.
Color depth is measured by counting the total number of colors recorded or displayed.
Black and white photographs do not need to be scanned in color. Do not use settings intended for FAX or plain text documents. These do not have enough colors for photographs. Be aware that some antique photographs were tinted or have an overall color cast. To preserve the color in these images you should scan in color, not grayscale, for most purposes.