If you are an artist with an inspired design for a Liberace stamp sadly you cannot submit your work until the stamp is approved and the committee does not accept un-solicited work.
The good news is that you can write to the address below to start the process to get approved as an official artist BEFORE any work is considered.
From the official USPS site:
Artwork for Stamp Designs
Once a subject is approved, the Postal Service relies heavily on art directors under contract to the Postal Service for the selection of artists who will execute the designs. Stamp designing is an unusual art form requiring exacting skill in portraying a subject within very small dimensions. Due to the demands of stamp design and reproduction requirements, it is our policy not to review nor accept unsolicited artwork.
Professional artists who may wish to be considered for a design assignment should request a copy of the Creating U.S. Postage Stamps brochure from the following address:
U.S. Postal Service
ATTN: Stamp Design
1735 North Lynn Street, Suite 5013
Arlington, VA 22209-6432