The easiest way to do it is to resize you photo. Instead of 800×600 make it 640×480 for instance.

If you use Paint Shop Pro you can safe a file with more compression making the file smaller. You have to press “options” in the “save-as” screen. There you will find you can change the settings.

Photoshop can also do that, but I don’t know how as I don’t have that program.