I have an idea - I think that in addition to the thumbnail background color, there should also be a thumbnail border color. The width attribute would relate to the thumbnail border while the opacity attribute would relate to either both or just (more importantly) the background color.
This would help when setting the opacity of the image to something other than 100%. For example, I wanted my border color to be a greenish tint, although my images looked horrible with the green coming through at 80% opacity.
Thoughts?