On the contrary, 3rd party 2D programs are the basis of producing skin textures or any type of texture for that matter.  UVMapper would provide the 3D view to check the painting before bringing it into Poser.  Blacksmith3D will allow you to paint directly on the 3D model, but this is not the only program available. 

INJ/REM files can be created in various programs, but as Blackhearted mentions, they are not free.  However, there are some freebies which can convert these morphs easily after you spawn them in Poser.  Someone will come along with a listing.