I use CMD + Shift + 4 and SELECT what I want with the mouse. Then - it instantly puts that on my desktop.
Right-Click on the .png file created - and open with Preview. In Preview - File - Save As... Select .jpg - and the "quality" slider will change the file size (and quality obviously) You can also crop and what not in Preview.
or you download tinkertool which is a sort of add on to the system preferences. you can choose to save screen shots as jpegs automatically