how can I find/change a photo from a free theme

Hi. I uploaded a free theme I found that I really loved, but there’s a photo in the header that is totally wrong for the blog I’m setting up. I wanted to switch it out with another photo. But I can’t find where the header image is located. I read through the code in the stylesheet and the header and only saw this in the style: url(“images/header.jpg”). But that just takes you to google images with a header search. And none of those are the header image. The top part of the code in the style card is:

[CSS code moderated – please post a link to your site]

Does anyone have any ideas? Where does the program know to get the photo? I can’t figure it out! Even if I can’t get the photo and alter it, I’d love to know this for the future when I try and do my own.
Thank you,
Jen