Upload the wallpaper in your project, and tick the box that says “This has a custom price”, and set the price to $1.00. This will keep the game free, but anyone who has invested $1.00 or more on that project, will get access to that file.
Keep in mind this works in a “Tier” system. That means if one file is set to $1.00 and another file is set to $2.00, if a user invests $2.00 to your project, they will have access to both files.
Hope that’s what you were looking for.