Not sure about other countries, but this information is for the UK
You must ensure that any income received from selling these templates is declared to HM Revenue and Customs (previously the Inland Revenue) - (http://www.hmce.gov.uk) - via the Self Assessment (http://www.hmrc.gov.uk/sa/index.htm) method.
You should also look at the HMCE's "Setting up your Own Business" section - found at http://www.hmrc.gov.uk/businesses/iwtregister-a-new-business.shtml .
Registering a Limited Company in the UK is done via Companies House (http://www.companieshouse.co.uk/) - see the website for more information.
Supplying such templates online for free (no charge etc) would mean you don't have to register with Companies House or the HMCE...
If you do decide to sell them online, you may wish to have a contract which states that the user is not allowed to reproduce the templates, pass them off as their own design or resell them... You could also have a note at the bottom of the template saying something like "Copyright Tri-state betta 2006" with a link back to your site - if this is the case, you should ensure that any contract states that the user is not allowed to remove this line.
Also, if you are handling a User's personal details, you need to register with the Information Commissioner's Office (previously known as the Data Protection Agency) - more information is available on their website at http://www.ico.gov.uk
If you're non-UK, then this has little or no relevance whatsoever!