I am trying to use the paperclip plugin in my rails app to upload images for posts. I am using Rails 2.0.2 and ruby 1.8.7 on Ubuntu 10.04 os for project specific purposes.>
Ok, firstly, just to cover the "obvious" base: do "ls vendor/plugins" just to make sure that some version of paperclip is actually in your plugins directory and is therefore reachable by your application.
Secondly - the "require paperclip" in the model seems wrong to me. I'm guessing that require should go in your environment instead. Try putting it at the end of config/environments.rb instead.
Thirdly (and maybe unrelated to your specific problem) - 2.0.2 is a really old version of rails. If you can I'd upgrade to 2.3.5, or at the very least, 2.2.x. For all we know - you could have a problem because you've got a new version of paperclip and an old version of rails... this does sometimes make a difference.
A lot of the other links you posted are referring to installations up to Rails3 - the solutions to their problems will be different to yours for that reason.