Affiliate Marketing Secrets

I did a website layout for an organization that my organization was affiliated with. I did an initial layout for them in 2008 and had no issues. They requested changes to the website layout which involved me having to redo the site (old HTML style). They got the bill and voted to only pay half of it. After 2 months without any payment I took the new site down. They requested the 2008 site be put back up until they paid. 3 months after the original invoice due date a check for half the invoice amount was paid. I tried to put the new site back up but they locked me out and said they were taking over the site from there. I recently checked back with the site and they used my design, words/content, and replaced Designed By: Myself with Designed By Their Committee. Clearly they ONLY changed the header, but based off my original idea. Is this grounds for copyright infringement? Can I take legal action? If so, what do I do?
Screen shots here: http://www.flickr.com/photos/94823397@N00/4972516007

I did a website layout for an organization that my organization was affiliated with. I did an initial layout for them in 2008 and had no issues. They requested changes to the website layout which involved me having to redo the site (old HTML style). They got the bill and voted to only pay half of it. After 2 months without any payment I took the new site down. They requested the 2008 site be put back up until they paid. 3 months after the original invoice due date a check for half the invoice amount was paid. I tried to put the new site back up but they locked me out and said they were taking over the site from there. I recently checked back with the site and they used my design, words/content, and replaced Designed By: Myself with Designed By Their Committee. Clearly they ONLY changed the header, but based off my original idea. Is this grounds for copyright infringement? Can I take legal action? If so, what do I do?
Screen shots here: http://www.flickr.com/photos/94823397@N00/4972516007