How to convert the RGB images back to PCX and make high quality L and M tiles, so here it is:
Duplicate the finished image with all the layers and then flatten the image. If you get a pixelated look ingame with the finished H tile, then you can and apply a gaussian blur filter with a radius of 0.3 to prevent this.
Then save the image with the “Save for Web” command. Select GIF, Lossy: 0, Custom, Colors: 252 (256-4 night lighting colors), No Dither, no Transparency, no Interlaced, WebSnap: 0% and save the image. Open the saved GIF in PS and get the color table with “Image→Mode→Color Table…”. Unfortunately the positions for the night lighting are switched in Photoshop so you have to switch the positions by hand. Therefor click on the first color at the top left of the color table and hit CTRL-C to copy the already marked hex value for the color to the clipboard. Then click on one of the 4 spots you left blank as you saved only 252 colors. When the color dialog opens hit CTRL-V to paste the color value from the clipboard to change the color. Repeat this for the next 3 colors from the beginning, too. Now that you saved the first 4 colors into the last 4 blank squares you should change the first 4 colors to the ones you want to be your night lighting colors. Choose 4 colors for night lighting that aren't in your image already, instead choose new ones and paint the night lighting effects after converting to PCX. Finally save the color table so that you find it again. It doesn't matter if your GIF image looks a bit “strange”, we need this GIF only to get the color table right. After this you can close it again without saving.
Now get back to your duplicated and flattened image and convert it to indexed mode with “Image→Mode→Indexed Color…”. Choose “Custom” for palette and load your previous saved color table (the corrected one from the GIF). Set Dither to “None” and save the image as your H tile.
For the M tile take your original PSD file with all the layers and duplicate it again (like for the H tile). Then resize the image to 256 and apply a sharpen filter when sharpness is lost while resizing. Select Image→Mode→Indexed Color, say OK to merge the layers, then choose Custom for the palette and load your previously saved palette if it isn't already loaded. Also choose Matte: none and Dither: none. Then save the image with “Save As” as your M tile PCX.
Repeat the procedure for the M tile to get the L tile only resize to 128 instead of 256. If you want to scale down the H tile from 512 to 256 or 128 then first resize, apply a sharpen filter one or more times so that the image is quite sharp. Then apply the gaussian blur to it when needed and go on with the procedure described above.