Welcome aboard! Hope you ig your new site!
That's called the Splash screen. You can play around with it from your admin menu at
Admin > Layout > Splash Screen.
The file to upload is expected to be an HTML file, but conveniently, that particular HTML file can be simple (as in only a few lines).
To see example Splash pages, when you go to a splash page (like
http://www.dkpsystem.com ), right click on the page and click "View Source" or "View Page Source" and that will let you see the code used.
PS. I see you also have one of those
blasphemous signatures! you hurt me! You cut me deep!
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It's all in the reflexes.