I'm fairly new to rails, but I think I can help you.
I think you wan to use RMagick's from_blob flunction. It takes an
image as binary data and gives you an Magick Image object. Like so:
image = Magick::Image.from_blob(@params['picture_file'].read).first
Then you can manipulate it. For instance resample:
thumb = image.sample(200, 200)
Then write to file:
hope this helps