The easiest way is to use MS photoeditor, which comes with XP although it has to be loaded manually.
Firstly size your image to the required size. I tend to set my horizontal size to about 5cm (or less). You will determine you own best settings by trial and error.
Secondly make sure that the file size is a sensible limit. For most online images less than 100K is adequate. You can set this by using Save As, and setting the JPG quality to the required setting. You can check the saved file size and adjust if necessary then resave.
Note, it is best to save as using a new file name, so you dont edit the master image.
You can do all this using alternative image editting packages, they tend to come with cameras as standard now?