i would recommend getting the original default theme (K2), which has an image in it already and starting fresh! the custom theme you are using (vader, a variant of kubrick) has no img in it.
so go with the default kubrick theme. in it, they separate out the header img reference in a file located in a place like:
and you will be able to find the CSS at:
and it is controlled with this in the light.css:
background: url('/wp-content/themes/k2/invader/logo.png') center center no-repeat !important;
search in the light.css and look for the URL to the img for the header, and change it to where your image is.
here is more info on this:
ps- you probably noticed this, but you misspelled 'forum'
best of luck.