How do I use decals?
From Legacy Roblox Wiki
Decals aren't really even supported in the game yet. Use at your own risk.
- In Roblox Studio select the brick you want to add a Decal to.
- Select the menu option Insert -> Object
- When the dialog box pops up type "Decal" (no quotes)
- A Decal object will be placed under the brick in the Explorer tree view
- Change the Texture property of the Decal to be what you want
Note: Decals work best on maps when they are of the jpg type.