It is probably paint that does a poor job in optimizing the jpg file.

Go get the (free for personal use) faststone image viewer

Load up your file then go save as (a jpg again). Click on the "Advanced" button"
Seaport..Coastal City Isle of Sunderholm-2008-10-18_131644.png

THis will give you a screen where you can adjust all the jpg settings. The main one is the quality slider. After you change it, click the preview button. It will show in the top right window what the image will look like with that amount of compression, and at the bottom it will show what the new file size would be.
Seaport..Coastal City Isle of Sunderholm-2008-10-18_131828.png

-Rob A>