2 pixel wide lines render better in native player. Three pixel wide lines will work great too - but are much heavier visually.
Is there anything I can do to the image to update it and fix this issue?
Did you already fix it? It looks fine to me going through the history of the games linked to earlier in this thread.
What has worked for me is a single pixel border on the fill map and then the border drawn over on the board image however I want it to look.