They are available for personnal use only or for internet use ( eg : avatar, signature, etc ...).

For all others usages, prior agreement from the autor is needed, and you have to display copyright mention with a link to Mascot Factory
  Eg : Amator or personnal website, paper support, wears, etc ...

Commercial use is forbiden, except if you have explicit agreement from the autor
So you must not :

 - Use png in a software interface
  - Use png in a professional website layout
  - Sell or distribute those png
  - Use png for marketing product
  - etc... (nonexhaustive list)

By downloading a png file, you agreed with general Mascot Factory terms of use, according to autor wishes