I found a function getMap
in the rworldmap package, is that the one that you are using?
If so, it looks like it uses the methods from the sp package to do the plotting, these methods have an add
argument that when set to TRUE
will add the map to the current plot rather than starting a new plot. So try something like:
image.plot(...)
newmap <- getMap(resolution="high")
plot(newmap, add=TRUE)
and see if that works for you.
And in the future, please specify which packages your are using and give details that will help people help you.