Assuming you are selecting the icon that looks like a postcard (yellow with what looks like a mountain on it and a postage stamp in the upper right corner), you copy the link to the photo site (photo bucket, etc. ) and then in the two boxes that ask you to enter your pixel sizes, just enter 640 in the second box, skipping/leaving the first box blank).
It doesn't matter how big your original photo from your photo site is, as long as you enter just 640 in the second box, again leave the first box/space empty.
Try that.
Here's a link to the information I got -
link on photo posting size.
