Make sure you differentiate between the asset id and the decal id
The asset id is shown on the website and will not work directly as a decals id
The only way to currently convert asset ids to decal ids is through pasting the id through pasting the asset id into property menu directly in studio. Roblox will then convert this asset id into a decal id usable in a script
In the simplest terms, create an ImageLabel anywhere, paste rbxassetid://6878382428 in the Image property of ImageLabel, then Roblox would convert it into a different link. Copy the link, then replace your current BackgroundImage.Image with the new link.
Don’t forget to delete the ImageLabel afterwards - you don’t want it hanging around without a purpose.