Hi there,
when we upload image to your portfolio all is the same except padding in header.
So, open your style.css file
(you can find it here \wp-content\themes\astir\style.css)
on the line 1728 you have this code:
#astir_main_header.header_layout_left.smaller_header {
padding-top: 11px;
padding-bottom: 15px;
-webkit-transition: all 250ms ease-in-out;
-moz-transition: all 250ms ease-in-out;
-ms-transition: all 250ms ease-in-out;
-o-transition: all 250ms ease-in-out;
transition: all 250ms;
}
Replace
padding-top: 11px;
padding-bottom: 15px;
with
padding-top: 27px;
padding-bottom: 10px;
and when you scroll your header will look fine :)
Kind regards