STEP ONE
Open FastStone. In the left hand panel, browse to the folder where your image file is stored and select it with a left click

In the main panel, you will see thumbnails of the images in the folder. Double click on the image you wish to open

STEP TWO
With the image open, move your cursor to the left side of the screen and the main editing menu will open. Select Resize/Resample by clicking it or using the keyboard shortcut Ctrl +R

In the resize screen make sure Preserve Aspect Ratio is selected [1]. Next, change the pixel width to the recommended 640 [2]; you will notice that the height will change proportionately. Click ok [3].

STEP THREE
With your image now resized, you need to save it. Open the editing menu again and select Save As or type Ctrl +S.

Before actually saving the file, you need to set the level of compression to use. Click the Options button

STEP FOUR
In the File Format Option screen, move the Quality slider to around 50% [1]. As you can see in the before and after pictures, the file size will be a good deal smaller, but the image will remain clear and sharp, provided it was to begin with. If it does look bad, increase the quality level, but only as far as needed. Once adjusted, select OK [2].

Back on the Save As screen, select where you wish to save the file [1], change its name, if need be [2], and click Save [3]

Your image is now ready to upload to Photobucket or whichever site you are using to host your picture.

It is worth noting here that while you can upload your images and edit them directly on several of the image hosting sites, I have found the tools provided quite confusing and the results often more guess work than actual user control. I highly recommend taking a small bit of time learning to do such things for yourself; the effort will repay you well.