Hi @RPronk,
Thanks for writing in!
First of all you have applied a negative margin incorrectly. Actually it works. You have inserted a negative top margin. Your perception of the margin is that it will stay on top of the image which is not the case.
Do you want something like this?

In that case, please edit your page back in Cornerstone and insert the following custom css in the settings tab, Settings > Custom CSS
.x-column.x-sm.x-1-3 {
position: relative;
}
.x-column.x-sm.x-1-3 .x-anchor.x-anchor-button.jb {
position: absolute;
top: 260px;
left: 50px;
margin: 0 auto;
z-index: 999;
}
We would loved to know if this has work for you. Thank you.