Question on theme layout.

I’m pretty new to this and am sort of just taking stabs at it. I’m modifying a theme to suit my preferences since starting one from scratch was way above my head.

I’m curious as to how I would go about putting an image on the left site of my site at the top (http://www.netbug.net). I would like to have an image to the left, overlapping the main blog border and extending a bit to the left (basically just a smaller image of a drawn character).

My problem is that I don’t know how/where that would be defined?

I know it’s pretty vague, but could someone try to point me in the right direction?

Thanks