Another way is to add a decal image in the Creator Marketplace: Go to Roblox.com and then under create section, you can choose Creations. And then go to Development items, you should see Decals. Just upload your own images. (Remember to follow the size and policy, some images will be rejected immediately like personal image.

Once you import the image, Studio assigns it a unique asset ID. Every texture or decal image that you create and import to Roblox must adhere to the Community Rules and Terms of Use. To add a texture or decal to a part or union: In the Explorer window, add a Texture or Decal to the part or union.

Every texture or decal image that you create and import to Roblox must adhere to the Community Rules and Terms of Use. To add a texture or decal to a part or union: In the Explorer window, add a Texture or Decal to the part or union. An empty texture or decal object displays on the part or union with orange outlining.

To add a decal to a Roblox game, shift the decal face to cover all sides of the surface you want it imposed on (Left, Right, Top, Bottom, Back, or Front). You can upload customized images to your game using Roblox Studio Asset Manager, but the game needs to be published for this to work.