I've just found the FAQs on JoomlArt's site for you. You're not allowed to remove their Designed by and replace with your company.
Can I remove JoomlArt Copyright?
Yes. You can edit or remove the JoomlArt copyright information at the bottom of the template. You can claim the copyright notice "All right reserved", however the claim on creation of the template, such as:"Designed by Your Company"...are not allowed. Allowed: Copyright � 2005 - 2006 Your Company. Developed by Your Company Not Allowed : Copyright � 2005 - 2006 Your Company. Designed by Your Company
Original Source:
http://www.joomlart.com/joomla_templ..._faqs/448.html
I'm just trying to help you out, I'm not attacking you so don't take this the wrong way.