Copy the Overview from the
Tutorial Mission into your own mission folder. The mission folder should contain the mission.sqm, Overview.htm, Briefing.htm, thepicture.jpg and anything else you want.
Replace the jpeg with your own. Make sure the sides are a factor of 2 in pixels, but it sounds like you have that bit sorted already.
It might be an idea to get the tute mission overview working in your own mission first, before you try to change anything.