Dave is correct on the diplay type, but you had other needs and with just changing the display type your image will be on the same line as your headers.
Wrap those headers in a div like so
HTML Code:
<div id="centerme"><h1>xxx</h1><h2>yyy</h2></div>
Another reason you should wrap those headers in a div, is because changing the display type to inline will now force the size of the header tags to be the actual width of the text, so adding a text-align of center to the headers won't align them correctly.
HTML Code:
h1, h2 {display: inline; margin-bottom: 0;}
div#centerme {text-align: center;}
Make sure your image is not in that div. Then just center your image as you normally would on the next line. I wouldn't recommend css for that unless you know what you are doing because FF & IE render differently.
