4-leaf clover is considered as a symbol of luck because it is not common. (3-leaf is the common variety).
Chimney sweep as a symbol of luck dates back to ordinances that "forced" home owners to have their chimneys swept regularly. If their house caught on fire they were still "lucky" and didn't get any additional trouble because they had followed the rules and had the chimney taken care of.
The pig was a symbol of fertility and prosperity and therefore desired as lucky.
I have never heard of a pickle on a Christmas tree. I still think it is an American folk tale. I have heard of the plum (or prune) which is a derivative from an altered song (Am Weihnachtsbaume da haengt 'ne Pflaume....) A prune hangs from the Christmas tree.....