How to Change the DarkMenu Background to Your Own Image
DarkMenu replaces the vanilla panorama with a static RPG-style background. If you want your own artwork instead, you do it through a resource pack, not through the mod's config.
Locating the texture path
Open your resource pack folder and mirror the vanilla GUI texture structure. DarkMenu overrides the main menu background texture, so your pack needs to place a file at the same path the mod uses for its background. The safest approach is to open the DarkMenu jar with an archive tool, find the background PNG it ships with, and copy that exact folder path into your own pack. Keep the same filename and .png extension.
Matching the resolution
Use an image with the same aspect ratio as the original so it does not stretch oddly. A wide, landscape-oriented image works best because the menu fills a widescreen window. If your image is a different shape, crop it rather than letting it squash.
Loading it
Place the pack above DarkMenu in the resource pack list so yours wins the conflict, then reload resources with the in-game keybind. If the background does not change, your path is wrong — compare it character by character against the path inside the mod jar. Remember that DarkMenu also changes the options background and button text, so a pack that only replaces the main background will leave those elements in the mod's RPG style.