In this thread you will find some of the large empty backdrops, that can be used for map making:
viewtopic.php?p=9429#p9429
Now If I remove the background my maps look like the pictures shown below. You can see that I sometimes cut out objects like single trees or houses but also larger ones like forrests, fields or even larger parts that contain everything. There is really no rule and it depends on the map style.
Sometimes you have to be more precise when cutting out objects and sometimes not so much. It also depends how they blend in with the background.
What kind of map do you plan to create?
This one is at 200% zoom (I guess due to the dimensions of the picture it is shown smaller in the forum so it is maybe shown at 170% or something like that

So it maybe looks not sharp. But you get the idea how a map of mine looks when the background is removed.):
This one is at 100% and you can see that there are some larger objects used that I cut out from the original SSI map. Sometimes it is best (or even the easiest way) to go back to the source, which are the original SSI png maps in most cases.:
