Hey there! That one almost got by us!
“Is it possible for one to upload their own header for this template? “
Sure! I think you could do that!
I think with the green apples theme, you can find the the header image in the “images” folder located within the theme folder itself. It is labeled green-apples.jpg. You can replace this image with a different one of similar (if not exact) size, and the same name. Or, you can choose your own image and adjust the image path/name by opening the style.css file in the green-apples theme folder, and editing this line to match your chosen image;
#header {
background: #fff url(images/green-apples.jpg) no-repeat bottom right;
border-bottom: 1px solid #a8b772;
}