Ok, now I see why. You are using the image as a “background-image” in your css, and I don’t know if its possible to make a background-image responsive. All of the images that you see that resize and are responsive are embedded in the code in one way or the other.
For example, you can make that same image responsive by adding it into your code this way –
However, the issue with doing it that way, is that it will be difficult to keep the elements inside (like the forms) to stay in place.
The best solution in my opinion is to set it up exactly like how they have it in the Generate theme by StudioPress…..(I’m assuming you already have it since you have the image). If you look at the Generate theme http://demo.studiopress.com/generate/, you’ll notice the image is in separate parts, and as you resize your browser some of those parts remove and are replaced with images that can repeat (giving the appearance that they are resizing). So if I were you I would do it the exact same way.
Let me know if this makes sense.