There are a few issues here.
One - the 95% layout - that's 95% of whatever screen resolution that's running. So if they have a 1940px resolution, that's about an 1843px width layout. They have a 800px resolution, you're gonna get a 760px layout.
Images are fixed size. They don't grow or shrink. The way I imagine this template handles it (and I don't really know) is that you'd need to put a really long image into the header, and it would just use the required percentage width of that header image depending on the screen resolution. So if you had a header image 2000px x 200 px, only 80 px on each side would not be showing in my original example, but only the middle third of the image would show in the latter example.
I hope that's right. I can't imagine how else to deal with it. That''s why I never use percentage based layouts. I just decide on a width and leave it at that.
Hope that helps.