Put a background color behind header image (newbie)

I want my preferred blue (#A6ADDB) to appear behind images so that certain images which have transparent areas will show this blue instead of the default white.
In Sept. 2009, juggledad wrote a solution for a background color behind the image (I think). That is what I want to do rather than have the blue showing in the main body by editing "Body, Text & Links". I was able to change Horizontal Bar 1: Styling to A6ADDB.
Where exactly do I place this code?
Wrap the code in a div like '<div class="gnec">.....</div>'
then use a css insert
HTML Code:
div.gnec {
background: #00ff00;
}

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