When your map becomes available online, there will be a little thumbnail graphic of the map. If you are curious what this thumbnail will look like before you upload your map, you can take a look. It actually gets written to disk at this location:
%userprofile%\Documents\creeperworld2_screens\thumb.jpg.
This file will get written out each time you save your map. So take a look at this image to see what your thumbnail will look like before you upload your map.