You can create your own clothing inventory items from textures made with Photoshop or Gimp. These clothing items can then be worn on your avatar and also shared with other users in the same way as other inventory items like models.
As well as building using the basic shape models provided by the building system, you can also import 3D models created in a 3D modeling application such as Blender or 3D Studio Max. There are also many sites on the internet where 3D content can be downloaded.
Image and texture files can be imported into your inventory for use in world building, avatar appearance and other content creation purposes. Imported textures can be used to create materials and clothing inventory items.