Board Thread:Support Requests - Designing Your Wikia/@comment-25523075-20150707124437/@comment-17725354-20150805153814

Those are just the edges of the layer, and it shouldn't show up when you export the image (make sure you always export the image rather than saving it, unless you are planning to work on it later). If you want to, you can select Layer to Image Size in the Layer menu so that the lines appear along the sides of the image.