Hook: Wall Small (Green Square)The Firebrand Graffiti decoration fits into a small green square wall hook.
Hook: Wall Medium (Blue Square)The Firebrand Graffiti decoration fits into a blue medium square wall hook.
How to Get a Firebrand Graffiti Decoration
Cartel Pack Decorations
The Firebrand Graffiti decoration can be dropped from Disavowed Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Disavowed Alliance Pack, include a random selection of items from the Disavowed Alliance collection of items. You aren't guaranteed to get a Firebrand Graffiti decoration from a Disavowed Alliance Pack, but it is one of the few ways to get a Firebrand Graffiti deco apart from buying it on the GTN from another player.
If you're looking to get a Firebrand Graffiti, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.