To install one that is posted here
Simply download the image by right-clicking (Windows) or Control+clicking (Macintosh) and then choosing Save As... or Save to Disk... and put it in the Textures / Decals folder.
Once it is there, you can apply it by bringing the object to which you want to apply the decal into the green room, once it's there...
1) Click the Color, Textures & State button/bar at the top right
2) Select the part to which you want to apply the decal from the Part Name drop-down menu
3) On the Macintosh, to the right of the Repeat Texture Slider is a small button labeled Decal. On Windows, it is half-way down on the dialog and it is a wide button labeled Apply/Edit Decals. Find it and click it.
4) The Texture Decals dialog will show up with the base object texture in it.
5) Click Choose Decal and navigate to the Texture/Decals Folder if neede
6) Select the decal you want to apply and position it as desired.
7) Click OK and voila...
NOTE: These "decals" are simply graphics with either an alpha mask (in the case of TGAs) or a transparent color (in the case of GIFs) and you can make them yourself in your favorite program. Please share any that you make in the new DECALS FOLDER on this site.