I don't know anything about this sort of thing so I won't attempt to answer the question directly. However, I will ask some subsidiary questions to which I hope the answer is yes.
The picture is in the same directory as your mission and overview files, yes?
the dimensions of the picture are both (length and breadth) a factor of 2 in pixels, aren't they (eg 128x64) ?
What exactly do you mean by won't work? Does it not appear at all or is it wonky?