can't get rid of blank space at top, new header image

i’m editing a theme with a header image at the top that you can customize under “appearances”, but i’d like to add my own header image that’s no where near as large as the standard one you can upload.
i removed the header image, which just left a huge empty space at the top of the page in its place. i’d like to put my smaller header image in and remove some of this space.
any thoughts?