If you are storing your pictures in the Pictures folder on your computer, just click on the picture you want and at the top it will say "edit", click on the edit button and a new window will come up, usually Windows Live Photo Gallery. Then just click on the picture and at the top it will say Properties, click on that and it will say re-size, click on that and it will let you re size your picture to a smaller size. You can play with it to find out what works best, but as I recall the maximum attachment size allowed is 500 KB per post.
That is what works for me.
Tom G.