First, create the image you want to use as the wallpaper. The image size must be 128 x 72 pixels. The image must be saved as a 4-bit indexed (16 color) bitmap (.BMP) file.
Launch iDmakeDL.
Click "Create new..." -> "Tamagotchi-iD L..." -> "Livingroom Wallpaper"
A black rectangle will appear in (roughly) the middle of iDmakeDL. Click the Load button that's just below it. In the file browser window, navigate to the folder containing your wallpaper bitmap image, and select it.
Back in iDmakeDL, click the Grab button to create a preview image of your wallpaper.
You can edit the catalog name, and some other details if you wish, or you can leave them at their default values.
Click the Save button up near the top right corner to save your wallpaper as a download file suitable for sending to your Tamagotchi-iD L.