Understanding file sizes, types and resolution   
Understanding resolution
Resolution refers to the sharpness and clarity of an image. The term is most often used to describe monitors, printers and bit-mapped graphic images.  The resolution indicates the number of dots per inch.  For example, a 300-dpi (dots per inch) printer is one that is capable of printing 300 distinct dots in a line 1 inch long. This means it can print 90,000 dots per square inch.

For our purposes, all images should be 72 dpi.  There is no need to have a higher resolution for anything that is intended to only be displayed on the screen, and the higher the dpi, the larger the file becomes, slowing down loading and display time.