hmmmm, nasty. There is a tute on this, as you probably saw. The most important thing is to save your picture with sides (in pixels) a factor of two - 2,4,8,16,32,64,128,256,512,etc.
If you have changed the picture you will also have to delete the old .pbo from the mission folder, open the mission editor, open the mission, save to single missions.
Getting the overview right is a real bore because of this process, but there is no way round it as far as I know. If your just trying to solve a picture problem it make make life easier to insert it in your briefing as a temporary measure, at least then you can test it from the mission editor (hold down the SHIFT key when you click on Preview.) If it works in the briefing it should work in the overview.
Try getting it to work with no picture at all, then at least you know the text is secure.