A few points:
1) Make sure that the maptemplate image has exactly the same name as the map you want to use it for. So, if the map name is "template", using the map file "template.xml", then the maptemplate image should be named "template.png". My guess, based on testing your image myself, is that this is your problem.
2) Supposedly, the game is quite picky about the size of the image- the image you posted is 510x510, not 512x512. That said, my version of the game (1.1.0 for MacOS) used it just fine.
3) When I tested the maptemplate, the game gave a warning about "png color depth ( < 3 bit) not supported", although this does not appear to have prevented it from working. What program did you use to make the image, and did you tell it to use any specific settings when saving?
4) The image should be pure black-and-white- yours has bits of grey along the endges of most of your lines. Once again, this didn't prevent the game from using the image, but I don't know just how it handles greys. If your image editor has a "convert to monotone" feature, using it might be a good idea, at least if you care about the pixel-level details of your map.
- Edwards