If the image is GENERIC (applicable to all sidebox headers), then you can apply a declaration to the div CLASS.
Code:
.leftBoxHeading {height: 30px;}

If you want to apply it to specific sidebox headings, then you'll need to use each sidebox's SPECIFIC div ID...

Code:
#categoriesHeading  {height: 30px;}
Using the latter will allow you to style each sidebox header differently, as I am doing HERE in this demo site.

You are more than welcome to inspect my styelsheet.css for this site, to see how I have styled the sidebox headers.

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