Hello,
Regarding your query, you can refer to the documentation link given below for detailed assistance.
https://themepalace.com/instructions/themes/blog-diary
To change the image below the header image you can add CSS following the path Admin Panel > Appearance > Customize > Additional CSS,
div#page-site-header {
background-image: url(Your Image URL) !important;
}
You can add the image URL you want to display in the ‘Your Image URL’ section.
Hope this helps.
Thank you.