As I understand it, Blockland doesn't support any animated textures, so the only way to do this would be to have a hacky script that cycles decals as if they were frames in a GIF.
A bigger problem is just with the way the decals work- you only see them if you've got the decal, so you can't exactly show it off to anybody without making them download the decal or decals, possibly even the script, too, if it's client-sided.