It might be worth pointing out that if you use Photoshop, make sure you use Save for Web instead of Save As when saving JPEGs. Not only do you get more specific compression options down to the percentage and all the comparison stuff, but it also chops out 10-100kb from the file size that's normally filled with superfluous meta data. Try getting a 800x600 jpeg and saving it twice as full quality with the same options under the two save modes and you'll see a difference of around 80kb.
A lot of people don't realise this, so don't feel patronised those who don't. D;