EDIT- Oh, wait, I just saw that you mean if you change the size of the window viewer. Welp, can't help you there, other than potentially one solution: measure the length of a full sized window in units of pixels, then add transparency to your image increasing the length of the image so that it matches the window size, but keep the actual picture in the center of the now larger image. For example, with the default view size you can put the image in the middle by making the width 670 pixels, with transparency on either side.
Obviously this will do little for arbitrarily sized viewer choices.
Hopefully the rest of this is useful.
You should be able to use on-page scripting for that. For example (using brackets instead of less-than/greater-than symbols):
[img src="INSERT IMAGE URL" width = "xxx" height = "yyy"]
If you want to move the picture to the center, you could use text align before posting the image script:
[p style="text-align:center][img src="INSERT IMAGE URL" width = "xxx" height = "yyy"][/style]