The physical size (not dimensions) of images you use on your web site must be considered. You cannot use an image that you took with your latest top-of-the-range camera as is on a web page - it is much too big in size, normally several Megabytes in size. The bigger the size, the longer the image (and your page) takes to load, which is frustrating for the user and wastes the bandwidth he/she has available. Considering the cost and size of data bandwidth, you should optimize your images so that they are the smallest size possible whilst maintaining a desired quality.
The following tutorial shows you how to achieve this goal with Paint .Net, the image editor on our CD.
http://www.small-business-website-designz.com/paint.net.shtml
We will not be strict on this when marking the assignment, but for the portfolio we will! In general terms, and it is an unwritten rule, you should strive to keep an image under 100 Kb. That said, if you have 12 images of 100 Kb each on a single page, that amounts to 1.2 MB! So another unwritten rule is that you should only add images if they add value to a page.