When submitted, your decal must be reviewed before it is available. You can create decals by going to the "Decals" tab under the Develop section. Once you have reached there, use the "Choose File" button to find the picture you want to use. It will add the decal to your account as soon as you click "Upload".
On May 14, 2008, Matt Dusek announced support for user-created Decals. Decals could be created easily now with the Decal Builder and they had their own category in the Catalog. Since then, Decals have been moved to the "Develop" section, as well as being moved from the Catalog to the Library.
Use in hacking
When exploiters use decals, they would usually place a decal on one of the bricks and then put the same decal on every brick in the game has so it would lag, which often occurs in popular games.