Jenkins Area Meetups love special Jenkins art for their meetup, and some plugins are important enough to deserve their own logo. You can check the gallery for some amazing contributions, or propose your own on the website repository.
Useful links:
Jenkins itself could also use some eye candy; dive into the Jenkins repository for places to start, or browse the issues on Jira
You can also propose your pixel-perfect css improvements and/or images for Jenkins.io on the website repository.