What if I want an image, right here?

Right here, people:

https://2img.net/r/ihimizer/img225/8744/fm1j.png

... Where the red line goes.

Assuming there is no need to complicate things you would add it to your homebox
It will be benith the "Current time" and "View last posts"

Not too sure about that. Because if I put the image in the box, more space will be used on the forum than necessary. Am I correct?

More space than necessary? It's a matter of the image size and your choice on how you want your site to appear. If that's where "you" want the image to display, then inserting it in your "home page message" as nera suggested, is how, or your easy option.