I'm quite bad at CSS, been spending the past hour fiddling to get an responsive image to align to center bottom of the page. This doesn't really do it, but it's one iteration out of many. The ones that get close are more frustrating, since it looks right until you resize it.
body{
width:100%;
height:100%;
}
.flexbox{
display:flex;
justify-content:center;
align-items:flex-end;
}
.alignbottomcenter{
width:50%;height:50%;
max-width:50%;max-height:50%;
display:flex;
justify-content:center;
align-items:flex-end;
}