Personally I wouldn't bother downloading anything as you will already have Windows "Paint" installed on your computer and to do it it couldn't be simpler...
Open Paint by going to the start menu-all programs-accessories-paint...... when loaded import the photograph you want by clicking on the left hand blue tab and choosing the open option, now navigate to the pic you want and double click on it.
You will be faced with something like this
Now click on the resize button at the top and in the drop down click pixels, now you have two figures below this which are the Horizontal and Vertical sizes you just need to adjust the greater of these to 700* and click OK.
Now the image will have reduced and simply once again click the top left blue file tab and select "save" or "Save As" if you want to make a copy but keep the original
*The 700 pixel size is based on starting with a 12mp image you may have to adjust this for larger or smaller mp sizes (smaller for smaller and larger for larger)