Update 2006-12-31: I don’t think this is quite right. It keeps creating a folder in my Rails root as well as in public/. I think I might switch to acts_as_attachment anyway.
In your model, where you define the sizes and whatnot that you want, put:
file_column :database_column_name,
:store_dir => 'path/you/want',
:base_url => 'path/you/want',
(the rest of your configuration)
So what you end up with is images going into rails_root/public/path/you/want/database_id_of_image/