First download latest ServerGridEditor, it was updated.
My maps are working with filenames as created from ServerGridEditor (MapImg.jpg, CellImg_X_X.jpg)
Make sure, that also SeamLess data port is redirected (usually 27000).
btw, ServerGridEditor has still bug, that if you delete MapImageURL, it stays there. Changing is possible, deleting only manually by editing file in editor.
If still in trouble, change "MetaWorldURL": "" to any web server (could be also in your lan) like http://ip/GridServer and upload images there.