Home Page Header Images

Site I am developing calprogramsautistic.org I am learning WP and the Catch Everest Theme. I have multiple questions posted. 1. I wish to add an img with hyperlink to just the homepage header - right side. The image I wish to use is found in the right sidebar of all other pages - fundraiser image. When I add this image in the child theme header.php file and I resized the browser window to 1024 x 768 the image is partially cut off and the header image and the fundraiser image are not scaling. When viewed on a mobile device such as iphone the header logo appears at the top and the fundraiser image immediately below that. 1a. Did I place the image in the wrong container or what should have been done different to maintain responsive images. 1b.I think the fixed size noted in number 2 is a concern. If this needs to be manually edited since the theme implemented this size, what file php contains the header image html? 2. I also note in the theme header the header image is added with html height and width which seems counter intuitive if the theme is responsive. I used FF Web Dev Tool bar and inspector on the img tag in the #site-logo and the img file shows up with width 769 x height 125. 2a. Why are dimensions used in the theme html for header img. 2b. Second why does this appear to be responsive even with the dimensions. I tested this on my iPhone portrait and it still works but I would like to understand the theme better.