Re: multi image header
First off id suggest making a child theme, more info here https://codex.buddypress.org/theme-development/building-a-buddypress-child-theme/
You can put the html code in the header.php found in your theme folder, im not sure where you want it placed exactly so you will have to trail and error untill you find the right position.
The corresponding css code should be put in the _inc/css/deafult.css file, OR you could make a new separate css file and include this in your main css file like so
` @import url( _inc/css/your_new_file.css );`
