Since you know the size just round it off and use some CSS rules.
So
[IMG width=980]some image[/IMG]
Will output
<IMG src="some image" style="width:980px;" class="IMG_width1000" />
You can still define the main width and the class will override it with different viewport settings.
*edit, just reread my post, but in this example if the width is smaller than 1000 px override it to a percent
It is late and I am on mobile, I do it here.
http://www.sicomm.us/media/item/14/
The CSS is the minimalism-chat.CSS or similar file name.
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