Images for the web are typically created with a resolution of 72-96 dpi because they are targeted for monitors, not printers.
This is very important in terms of the size of the image file. In fact, an image at 300 dpi requires over 17 times more storage space than the same image at 72 dpi:
300 dpi = 90,000 storage locations per inch (300 x 300)
72 dpi = 5,184 storage locations per inch (72 x 72)
Bottom line? When you create images (graphics, photographs, etc.) for the web, be sure you save them at a resolution of 72-96dpi. Higher resolutions take significantly longer to download!