Steps listed below detail how to upload a decal.
1. Go to the Roblox Website.
2. Click on the Create Tab.
3. Click the Decals Tab.
4. Click Choose File and Upload your file.
Note: Roblox accepts .png .jpg .tga or .bmp image formats
5. Clock on the file and wait for the image to be moderated by Roblox.
For more information please follow this tutorial.