hey bohemi!
"lay options" -> "custom css & html" -> custom html at top
<a class="close-button" href="http://boe2.de/arbeiten/"><img src="http://boe2.de/wp-content/uploads/2017/03/Button_rechts_cross_v2.png"/></a>
"custom <head> content":
<style>
.close-button{
display: none;
}
body.type-project .close-button{
display: block;
position: fixed;
top: 20px;
left: 20px;
z-index: 10;
width: 20px;
height: 20px;
}
.close-button img{
width: 100%;
}
</style>
This is a simple example, I admit to do this properly you would need some html and css experience as this is not a standard feature by lay theme but probably it should be somehow.