Como Se Llama?

I'm in a current small scale legal battle over my T-shirt design called, "Como se Llama" I came up with in 2005. I originally designed my shirt using clip art of a llama, but once sales started to pick up in 2010 I redesigned the shirt using my own art. My sales topped out around Christmas of 2012 and it was after that other cafepress members began copying my design. So I applied to copyright my design. Now this is tricky business, the phrase, "Como se Llama" alone is not something you can copyright, I can copyright my llama image. I feel there is a copyright infringement issue since it's the image of a llama and the phrase that makes the design unique and hopefully funny too. The copyright office does not protect slogans and phrase but what about artwork with a phrase as a combined piece? What's interesting is I received a cease and desist letter from the lawyers at Chic Fil-A in 2006 about a shirt I was selling that said, "Eat more vegetables" obviously they were not within the legal rights of the copyright provision to sue me, but they were certainly hoping to scare anyone who was naive to the provisions of the copyright laws. I feel I have more of a case than Chic Fil-A since my sales are being negatively impacted by other people copying my design. My, "Eat more veggies" shirt wasn't impacting Chic Fil-A's profits. But since other people are using the phrase and a different llama image, does that mean they are legally copying my design? Is this how Naked Castaway is able to basically compete Survivorman's format without any legal fallout?
Como se Llama