The theme is copied, the settings are saved, and the boot menu still shows the default background. Four things decide whether your wallpaper appears - work through them in order and one of them is your answer.
You did everything the tutorials said - theme copied, settings saved - and the boot menu still opens with the plain default background, or with a black screen where your wallpaper should be. In almost every case the wallpaper file itself is fine; one of four pointers is broken: the path in ventoy.json, the image name inside theme.txt, the image format, or the menu resolution. Check them in this order and you will find it.
ventoy/ventoy.json on the data partition and find the theme file value/ventoy/theme/mytheme/theme.txt, with forward slashes - a Windows-style path with backslashes never workstheme vs themes, or a renamed theme folder, is the single most common causeventoy folder is at the root of the data partition - if you created it inside another folder, or on the small VTOYEFI partition, Ventoy never sees itThe theme loads, but GRUB shows a plain or black background when the wallpaper line inside theme.txt does not resolve. Open the file and look at desktop-image: the name there must be the exact name of an image file in the same folder as theme.txt (or a path relative to it). Files named background.png in the theme but wallpaper.jpg in the text file - or the other way round - is the classic mismatch. Also check for a typo in the extension, and delete any second desktop-image line left over from the theme you copied.
GRUB's graphics mode can load PNG, JPG and TGA images. It cannot load BMP, GIF, WebP or HEIC files - a wallpaper saved in one of those formats is simply skipped, with no error message. Convert the image to PNG (safest) or JPG and keep the file name in theme.txt in sync. While you convert, also resize: an image far bigger than your menu resolution wastes load time, and a tiny image stretched across a big screen looks broken even though it loaded.
gfxmode is 1024x768 - if your wallpaper is 1920x1080 and this setting was never changed, the menu runs at the small resolution and the image is scaled or cropped"gfxmode": "1920x1080" in the theme block of ventoy.json to match a Full HD wallpaper, or use "max" to take the highest resolution the machine offersgfxmode at the same resolution, and test on your oldest machine, not your newestThe video shows the exact folders and files this fix keeps referring to.
Watch Full VideoUsually resolution. If gfxmode asks for a resolution the second machine's firmware does not offer, GRUB falls back to a lower mode or a plain background. Setting gfxmode to max, and sizing the wallpaper to a common resolution like 1920x1080, makes the result consistent across machines.
The image resolution and the menu resolution differ, and GRUB stretches the image to fill the screen. Export your wallpaper at exactly the gfxmode resolution you set in ventoy.json - same width and height - and it will display pixel for pixel.
Only after converting it. Phone photos are often HEIC or very large JPG files. Convert to PNG or a resized JPG at your menu resolution, name it clearly, and make sure the desktop-image line in theme.txt uses that exact name.