Please...you are in a forum that provides support. EVERY question is either dumb or inane....sorry, had to get that off my chest. You know the right answer to that part of your question.
The answer to your real question is....tah dah....you have to change the style.css file in the theme folder for Kubrick, which is called "default" and it is found under
Open the file and do a FIND (cntrl+F usually works) in Notepad, or whatever text editor comes up, for the file name you want to change. When you find it, change it to the one you want. Then look for the other one, the same way. (if it says not found, send it UP instead of DOWN)
Save the file and upload it to the above listed directory on your host server. Done. Fixed.
Almost fixed. Now, you want to let the individual posts stay as the k-wide graphic. That is a little more complicated and I'm going to let someone else answer that part of the question because it means calling a different header file for the two different index and single post files....Or writing an IF/ELSE statement to change the look of your header dependent upon what is showing. That's a lot more complicated.
I hope the first part helps.