You mean a decal for Blockland?
As in the shirts that the blockheads wear?
If Yes, yes you can, however buying the game isn't required for that. It's all done outside of Blockland and all you actually require is an image-editing program to make the decal in, such as MSPaint, Paint.net, Photoshop, etc...
Then it's simply a matter of packaging it correctly. I believe there's a tutorial on it around here somewhere.
