Decriminalising graffiti will help graffiti artists to show and promote their identities. Having a law against graffiti suppresses and forces artists to go underground due to the fear of being arrested. For example, the artist Bansky is world known for his work yet no one knows his identity and many authorities want to charge him with offences when many people praise his talent. With This in mind, we need to help other artists achieve opportunities in pathways in the arts and in the creative industry rather than being hunted down and charged with a criminal record.
Graffiti on public properties should not be made legal, but they are artists in their own way. They should be recognised, and provided benefits. They do have talent, creativity, and can campaign effectively for a cause. Their courage and zeal should be used in a better way, for the society.
If you graffiti your own wall, yes, it should be legal. If you graffiti public buildings and private property, that's vandalism. As for the payed part, I don't know who would pay them. I guess people could hire others to paint a wall for them. The question is unclear and terrible.
Definition of graffiti: "Writing or drawings scribbled, scratched, or sprayed illicitly on a wall or other surface in a public place." Whoever supports vandalism must be an idiot, a criminal, or both. However, if the graffiti is done on your own wall and does not contain anything inappropriate, than I think it should be okay.