You can add some CSS to push the page down a little so that it isn't hidden behind the solid header, but the header and its contents are dynamically sized so you'll need to take care to ensure that it doesn't leave a gap at different 'viewports' (screen widths).
Here's a simplified example that you can add to Design > Custom CSS:
.homepage #siteWrapper {
margin-top:77px;
}
As you adjust your browser width from really small (mobile size) to really large (a 5K display) you'll see the issue.
There are various ways to account for this. For example, instead of the code above, you can use the following example instead, which adjusts the margin for different viewports using 'media queries':
@media only screen and (min-width: 800px) {
.homepage #siteWrapper {
margin-top:66px;
}
}
@media only screen and (min-width: 1280px) {
.homepage #siteWrapper {
margin-top:75px;
}
}
I hope this helps.
On Squarespace 7.1, the page sits 'behind' the header which is why part of the image is hidden. Do you want to change this behaviour on just this page, or others pages too?