wingsgb - the center tag is deprecated and should no longer be used.
punchline - an img is an inline element, and so can be centred by using text-align:center. You can set this on the enclosing h1 element here. It looks like you may have attempted to centre the element using margin:auto. This will work on a block level element if that element has a declared width. (Info on inline and block elements here
There are a number of other problems with your code, however, which are likely to cause problems later. Use the W3C validator to find these, and check as you go along (see link in my sig). Most importantly, your code does not have a valid doctype