I just changed the style of the post page to unboxed, which removed the gap between the navigation bar and the post. That allowed me to insert the image in that exact spot where the gap was. It’s a bit annoying that my post pages aren’t boxed anymore, but it’s a small price to pay. It’s more of a workaround than a solution really.
Nevermind, I was able to find a workaround 🙂
I disabled all the plugins first and checked before switching themes, turns out it was a plugin issue. Thank you so much, really appreciate it!