What i learned until now, u can spawn billboard ads, but just in the near where they normaly appear in the world. Another thing i noticed, reflections doesn't work for manually spawnded billboard ads.
So you need an exact name/hash for a billboard to spawn (with Menyoo ode Map Editor). Since te Mod "Map Info" is outdated, there is a nice tool called "Code Walker", which can load the GTA Map and let u Fly around in 3D. You can get detailed information about every object, like the exact name.
If you want to place only one personalized billboard, u can use this sign for example. It should be load everywhere you are and is much easier to place. A billboard and a billboard ad are two different objects which you have to attach and align manualy.