I was struggling for some time with issue of not knowing how to transform day background of my 2D maps into night one. Thanks to this article How Ben Adds Light and Shadow to His Dungeons I managed to create one, so decided to share this info here. You will need the GIMP tool. Open your day background map in GIMP; Context menu on base layer and "Add alpha Channel" if not yet added; Add second layer. Fill second layer with transparent dark blue color by first selecting Tools\Paint Tools\Bucket Fill In Bucket Fill properties pane (leftwards) choose Color: 0f1115, Mode: Darken only, and then fill whole section. Select Paint Tools\Eraser, in properties - brush: circle, radius: 340 (subjective), Force: 23 (subjective), Apply Jitter 0.2, Incremental and clear light areas. In original article Ben has even better techniques, but this suits me.