save the file to the location you want and then change the type of picture you are saving it as in the drop down menu

try saving a pic in jpeg and then in bmp format and see how much difference there is between the 2 of them

also, if you want to shrink the pic in paint, press ctrl "W" and it will allow you to stretch/skew the picture

just put in 30% vaules in the top 2 boxes and it will shrink the pic down from possibly 3mb to about 50 kb